# SQL Syntax

This chapter describes the syntax of SQL. It forms the foundation for understanding the following chapters which will go into detail about how SQL commands are applied to define and modify data.

We also advise users who are already familiar with SQL to read this chapter carefully because it contains several rules and concepts that are implemented inconsistently among SQL databases or that are specific to PostgreSQL.

## Lexical Structure

SQL input consists of a sequence of commands. A command is composed of a sequence of tokens, terminated by a semicolon (;). The end of the input stream also terminates a command. Which tokens are valid depends on the syntax of the particular command.

A token can be a key word, an identifier, a quoted identifier, a literal (or constant), or a special character symbol. Tokens are normally separated by whitespace (space, tab, newline), but need not be if there is no ambiguity (which is generally only the case if a special character is adjacent to some other token type).

For example, the following is (syntactically) valid SQL input:

    SELECT * FROM MY_TABLE;
    UPDATE MY_TABLE SET A = 5;
    INSERT INTO MY_TABLE VALUES (3, 'hi there');

This is a sequence of three commands, one per line (although this is not required; more than one command can be on a line, and commands can usefully be split across lines).

Additionally, comments can occur in SQL input. They are not tokens, they are effectively equivalent to whitespace.

The SQL syntax is not very consistent regarding what tokens identify commands and which are operands or parameters. The first few tokens are generally the command name, so in the above example we would usually speak of a SELECT, an UPDATE, and an INSERT command. But for instance the `UPDATE` command always requires a SET token to appear in a certain position, and this particular variation of `INSERT` also requires a VALUES in order to be complete. The precise syntax rules for each command are described in reference.

### Identifiers and Key Words

Tokens such as SELECT, UPDATE, or VALUES in the example above are examples of key words, that is, words that have a fixed meaning in the SQL language. The tokens MY_TABLE and A are examples of identifiers. They identify names of tables, columns, or other database objects, depending on the command they are used in. Therefore they are sometimes simply called names. Key words and identifiers have the same lexical structure, meaning that one cannot know whether a token is an identifier or a key word without knowing the language. A complete list of key words can be found in sql-keywords-appendix.

SQL identifiers and key words must begin with a letter (`a`-`z`, but also letters with diacritical marks and non-Latin letters) or an underscore (`_`). Subsequent characters in an identifier or key word can be letters, underscores, digits (`0`-`9`), or dollar signs (`$`). Note that dollar signs are not allowed in identifiers according to the letter of the SQL standard, so their use might render applications less portable. The SQL standard will not define a key word that contains digits or starts or ends with an underscore, so identifiers of this form are safe against possible conflict with future extensions of the standard.

The system uses no more than NAMEDATALEN-1 bytes of an identifier; longer names can be written in commands, but they will be truncated. By default, NAMEDATALEN is 64 so the maximum identifier length is 63 bytes. If this limit is problematic, it can be raised by changing the NAMEDATALEN constant in `src/include/pg_config_manual.h`.

Key words and unquoted identifiers are case-insensitive. Therefore:

    UPDATE MY_TABLE SET A = 5;

can equivalently be written as:

    uPDaTE my_TabLE SeT a = 5;

A convention often used is to write key words in upper case and names in lower case, e.g.:

    UPDATE my_table SET a = 5;

There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes (`"`). A delimited identifier is always an identifier, never a key word. So `"select"` could be used to refer to a column or table named select, whereas an unquoted `select` would be taken as a key word and would therefore provoke a parse error when used where a table or column name is expected. The example can be written with quoted identifiers like this:

    UPDATE "my_table" SET "a" = 5;

Quoted identifiers can contain any character, except the character with code zero. (To include a double quote, write two double quotes.) This allows constructing table or column names that would otherwise not be possible, such as ones containing spaces or ampersands. The length limitation still applies.

Quoting an identifier also makes it case-sensitive, whereas unquoted names are always folded to lower case. For example, the identifiers `FOO`, `foo`, and `"foo"` are considered the same by PostgreSQL, but `"Foo"` and `"FOO"` are different from these three and each other. (The folding of unquoted names to lower case in PostgreSQL is incompatible with the SQL standard, which says that unquoted names should be folded to upper case. Thus, `foo` should be equivalent to `"FOO"` not `"foo"` according to the standard. If you want to write portable applications you are advised to always quote a particular name or never quote it.)

A variant of quoted identifiers allows including escaped Unicode characters identified by their code points. This variant starts with `U&` (upper or lower case U followed by ampersand) immediately before the opening double quote, without any spaces in between, for example `U&"foo"`. (Note that this creates an ambiguity with the operator `&`. Use spaces around the operator to avoid this problem.) Inside the quotes, Unicode characters can be specified in escaped form by writing a backslash followed by the four-digit hexadecimal code point number or alternatively a backslash followed by a plus sign followed by a six-digit hexadecimal code point number. For example, the identifier `"data"` could be written as

    U&"d\0061t\+000061"

The following less trivial example writes the Russian word slon (elephant) in Cyrillic letters:

    U&"\0441\043B\043E\043D"

If a different escape character than backslash is desired, it can be specified using the `UESCAPE`
clause after the string, for example:

    U&"d!0061t!+000061" UESCAPE '!'

The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character. Note that the escape character is written in single quotes, not double quotes, after `UESCAPE`.

To include the escape character in the identifier literally, write it twice.

Either the 4-digit or the 6-digit escape form can be used to specify UTF-16 surrogate pairs to compose characters with code points larger than U+FFFF, although the availability of the 6-digit form technically makes this unnecessary. (Surrogate pairs are not stored directly, but are combined into a single code point.)

If the server encoding is not UTF-8, the Unicode code point identified by one of these escape sequences is converted to the actual server encoding; an error is reported if that's not possible.

### Constants

There are three kinds of implicitly-typed constants in PostgreSQL: strings, bit strings, and numbers. Constants can also be specified with explicit types, which can enable more accurate representation and more efficient handling by the system. These alternatives are discussed in the following subsections.

#### String Constants

A string constant in SQL is an arbitrary sequence of characters bounded by single quotes (`'`), for example `'This is a string'`. To include a single-quote character within a string constant, write two adjacent single quotes, e.g., `'Dianne''s horse'`. Note that this is not the same as a double-quote character (`"`).

Two string constants that are only separated by whitespace with at least one newline are concatenated and effectively treated as if the string had been written as one constant. For example:

    SELECT 'foo'
    'bar';

is equivalent to:

    SELECT 'foobar';

but:

    SELECT 'foo'      'bar';

is not valid syntax. (This slightly bizarre behavior is specified by SQL; PostgreSQL is following the standard.)

#### String Constants with C-Style Escapes

PostgreSQL also accepts escape string constants, which are an extension to the SQL standard. An escape string constant is specified by writing the letter `E` (upper or lower case) just before the opening single quote, e.g., `E'foo'`. (When continuing an escape string constant across lines, write `E` only before the first opening quote.) Within an escape string, a backslash character (`\`) begins a C-like backslash escape sequence, in which the combination of backslash and following character(s) represent a special byte value, as shown in sql-backslash-table.

#### Backslash Escape Sequences

Backslash Escape Sequence | Interpretation
`\b` | backspace
`\f` | form feed
`\n` | newline
`\r` | carriage return
`\t` | tab
`\`o``, `\`oo``, `\`ooo`` (`o` = 0–7) | octal byte value
`\x`h``, `\x`hh`` (`h` = 0–9, A–F) | hexadecimal byte value
`\u`xxxx``, `\U`xxxxxxxx`` (`x` = 0–9, A–F) | 16 or 32-bit hexadecimal Unicode character value

Any other character following a backslash is taken literally. Thus, to include a backslash character, write two backslashes (`\\`). Also, a single quote can be included in an escape string by writing `\'`, in addition to the normal way of `''`.

It is your responsibility that the byte sequences you create, especially when using the octal or hexadecimal escapes, compose valid characters in the server character set encoding. A useful alternative is to use Unicode escapes or the alternative Unicode escape syntax, explained in sql-syntax-strings-uescape; then the server will check that the character conversion is possible.

If the configuration parameter guc-standard-conforming-strings is `off`, then PostgreSQL recognizes backslash escapes in both regular and escape string constants. However, as of PostgreSQL 9.1, the default is `on`, meaning that backslash escapes are recognized only in escape string constants. This behavior is more standards-compliant, but might break applications which rely on the historical behavior, where backslash escapes were always recognized. As a workaround, you can set this parameter to `off`, but it is better to migrate away from using backslash escapes. If you need to use a backslash escape to represent a special character, write the string constant with an `E`.

In addition to `standard_conforming_strings`, the configuration parameters guc-escape-string-warning and guc-backslash-quote govern treatment of backslashes in string constants.

The character with the code zero cannot be in a string constant.

#### String Constants with Unicode Escapes

PostgreSQL also supports another type of escape syntax for strings that allows specifying arbitrary Unicode characters by code point. A Unicode escape string constant starts with `U&` (upper or lower case letter U followed by ampersand) immediately before the opening quote, without any spaces in between, for example `U&'foo'`. (Note that this creates an ambiguity with the operator `&`. Use spaces around the operator to avoid this problem.) Inside the quotes, Unicode characters can be specified in escaped form by writing a backslash followed by the four-digit hexadecimal code point number or alternatively a backslash followed by a plus sign followed by a six-digit hexadecimal code point number. For example, the string `'data'` could be written as

    U&'d\0061t\+000061'

The following less trivial example writes the Russian word slon (elephant) in Cyrillic letters:

    U&'\0441\043B\043E\043D'

If a different escape character than backslash is desired, it can be specified using the `UESCAPE`
clause after the string, for example:

    U&'d!0061t!+000061' UESCAPE '!'

The escape character can be any single character other than a hexadecimal digit, the plus sign, a single quote, a double quote, or a whitespace character.

To include the escape character in the string literally, write it twice.

Either the 4-digit or the 6-digit escape form can be used to specify UTF-16 surrogate pairs to compose characters with code points larger than U+FFFF, although the availability of the 6-digit form technically makes this unnecessary. (Surrogate pairs are not stored directly, but are combined into a single code point.)

If the server encoding is not UTF-8, the Unicode code point identified by one of these escape sequences is converted to the actual server encoding; an error is reported if that's not possible.

Also, the Unicode escape syntax for string constants only works when the configuration parameter guc-standard-conforming-strings is turned on. This is because otherwise this syntax could confuse clients that parse the SQL statements to the point that it could lead to SQL injections and similar security issues. If the parameter is set to off, this syntax will be rejected with an error message.

#### Dollar-Quoted String Constants

While the standard syntax for specifying string constants is usually convenient, it can be difficult to understand when the desired string contains many single quotes, since each of those must be doubled. To allow more readable queries in such situations, PostgreSQL provides another way, called dollar quoting, to write string constants. A dollar-quoted string constant consists of a dollar sign (`$`), an optional tag of zero or more characters, another dollar sign, an arbitrary sequence of characters that makes up the string content, a dollar sign, the same tag that began this dollar quote, and a dollar sign. For example, here are two different ways to specify the string Dianne's horse using dollar quoting:

    $$Dianne's horse$$
    $SomeTag$Dianne's horse$SomeTag$

Notice that inside the dollar-quoted string, single quotes can be used without needing to be escaped. Indeed, no characters inside a dollar-quoted string are ever escaped: the string content is always written literally. Backslashes are not special, and neither are dollar signs, unless they are part of a sequence matching the opening tag.

It is possible to nest dollar-quoted string constants by choosing different tags at each nesting level. This is most commonly used in writing function definitions. For example:

    $function$
    BEGIN
        RETURN ($1 ~ $q$[\t\r\n\v\\]$q$);
    END;
    $function$

Here, the sequence `$q$[\t\r\n\v\\]$q$` represents a dollar-quoted literal string `[\t\r\n\v\\]`, which will be recognized when the function body is executed by PostgreSQL. But since the sequence does not match the outer dollar quoting delimiter `$function$`, it is just some more characters within the constant so far as the outer string is concerned.

The tag, if any, of a dollar-quoted string follows the same rules as an unquoted identifier, except that it cannot contain a dollar sign. Tags are case sensitive, so `$tag$String content$tag$` is correct, but `$TAG$String content$tag$` is not.

A dollar-quoted string that follows a keyword or identifier must be separated from it by whitespace; otherwise the dollar quoting delimiter would be taken as part of the preceding identifier.

Dollar quoting is not part of the SQL standard, but it is often a more convenient way to write complicated string literals than the standard-compliant single quote syntax. It is particularly useful when representing string constants inside other constants, as is often needed in procedural function definitions. With single-quote syntax, each backslash in the above example would have to be written as four backslashes, which would be reduced to two backslashes in parsing the original string constant, and then to one when the inner string constant is re-parsed during function execution.

#### Bit-String Constants

Bit-string constants look like regular string constants with a `B` (upper or lower case) immediately before the opening quote (no intervening whitespace), e.g., `B'1001'`. The only characters allowed within bit-string constants are `0` and `1`.

Alternatively, bit-string constants can be specified in hexadecimal notation, using a leading `X` (upper or lower case), e.g., `X'1FF'`. This notation is equivalent to a bit-string constant with four binary digits for each hexadecimal digit.

Both forms of bit-string constant can be continued across lines in the same way as regular string constants. Dollar quoting cannot be used in a bit-string constant.

#### Numeric Constants

Numeric constants are accepted in these general forms:

    digits
    digits.digitse+-digits
    digits.digitse+-digits
    digitse+-digits

where `digits` is one or more decimal digits (0 through 9). At least one digit must be before or after the decimal point, if one is used. At least one digit must follow the exponent marker (`e`), if one is present. There cannot be any spaces or other characters embedded in the constant, except for underscores, which can be used for visual grouping as described below. Note that any leading plus or minus sign is not actually considered part of the constant; it is an operator applied to the constant.

These are some examples of valid numeric constants:

    42
    3.5
    4.
    .001
    5e2
    1.925e-3

Additionally, non-decimal integer constants are accepted in these forms:

    0xhexdigits
    0ooctdigits
    0bbindigits

where `hexdigits` is one or more hexadecimal digits (0-9, A-F), `octdigits` is one or more octal digits (0-7), and `bindigits` is one or more binary digits (0 or 1). Hexadecimal digits and the radix prefixes can be in upper or lower case. Note that only integers can have non-decimal forms, not numbers with fractional parts.

These are some examples of valid non-decimal integer constants:

    0b100101
    0B10011001
    0o273
    0O755
    0x42f
    0XFFFF

For visual grouping, underscores can be inserted between digits. These have no further effect on the value of the constant. For example:

    1_500_000_000
    0b10001000_00000000
    0o_1_755
    0xFFFF_FFFF
    1.618_034

Underscores are not allowed at the start or end of a numeric constant or a group of digits (that is, immediately before or after the decimal point or the exponent marker), and more than one underscore in a row is not allowed.

A numeric constant that contains neither a decimal point nor an exponent is initially presumed to be type `integer` if its value fits in type `integer` (32 bits); otherwise it is presumed to be type `bigint` if its value fits in type `bigint` (64 bits); otherwise it is taken to be type `numeric`. Constants that contain decimal points and/or exponents are always initially presumed to be type `numeric`.

The initially assigned data type of a numeric constant is just a starting point for the type resolution algorithms. In most cases the constant will be automatically coerced to the most appropriate type depending on context. When necessary, you can force a numeric value to be interpreted as a specific data type by casting it.
For example, you can force a numeric value to be treated as type `real` (`float4`) by writing:

    REAL '1.23'  -- string style
    1.23::REAL   -- PostgreSQL (historical) style

These are actually just special cases of the general casting notations discussed next.

#### Constants of Other Types

A constant of an arbitrary type can be entered using any one of the following notations:

    type 'string'
    'string'::type
    CAST ( 'string' AS type )

The string constant's text is passed to the input conversion routine for the type called `type`. The result is a constant of the indicated type. The explicit type cast can be omitted if there is no ambiguity as to the type the constant must be (for example, when it is assigned directly to a table column), in which case it is automatically coerced.

The string constant can be written using either regular SQL notation or dollar-quoting.

It is also possible to specify a type coercion using a function-like syntax:

    typename ( 'string' )

but not all type names can be used in this way; see sql-syntax-type-casts for details.

The `::`, `CAST()`, and function-call syntaxes can also be used to specify run-time type conversions of arbitrary expressions, as discussed in sql-syntax-type-casts. To avoid syntactic ambiguity, the ``type` '`string`'` syntax can only be used to specify the type of a simple literal constant. Another restriction on the ``type` '`string`'` syntax is that it does not work for array types; use `::` or `CAST()` to specify the type of an array constant.

The `CAST()` syntax conforms to SQL. The ``type` '`string`'` syntax is a generalization of the standard: SQL specifies this syntax only for a few data types, but PostgreSQL allows it for all types. The syntax with `::` is historical PostgreSQL usage, as is the function-call syntax.

### Operators

An operator name is a sequence of up to NAMEDATALEN-1 (63 by default) characters from the following list:

    + - * / < > = ~ ! @ # % ^ & | ` ?

There are a few restrictions on operator names, however:

- `--` and `/*` cannot appear anywhere in an operator name, since they will be taken as the start of a comment.

- A multiple-character operator name cannot end in `+` or `-`, unless the name also contains at least one of these characters:

      ~ ! @ # % ^ & | ` ?

  For example, `@-` is an allowed operator name, but `*-` is not. This restriction allows PostgreSQL to parse SQL-compliant queries without requiring spaces between tokens.

When working with non-SQL-standard operator names, you will usually need to separate adjacent operators with spaces to avoid ambiguity. For example, if you have defined a prefix operator named `@`, you cannot write `X*@Y`; you must write `X* @Y` to ensure that PostgreSQL reads it as two operator names not one.

### Special Characters

Some characters that are not alphanumeric have a special meaning that is different from being an operator. Details on the usage can be found at the location where the respective syntax element is described. This section only exists to advise the existence and summarize the purposes of these characters.

- A dollar sign (`$`) followed by digits is used to represent a positional parameter in the body of a function definition or a prepared statement. In other contexts the dollar sign can be part of an identifier or a dollar-quoted string constant.

- Parentheses (`()`) have their usual meaning to group expressions and enforce precedence. In some cases parentheses are required as part of the fixed syntax of a particular SQL command.

- Brackets (`[]`) are used to select the elements of an array. See arrays for more information on arrays.

- Commas (`,`) are used in some syntactical constructs to separate the elements of a list.

- The semicolon (`;`) terminates an SQL command. It cannot appear anywhere within a command, except within a string constant or quoted identifier.

- The colon (`:`) is used to select slices from arrays. (See arrays.) In certain SQL dialects (such as Embedded SQL), the colon is used to prefix variable names.

- The asterisk (`*`) is used in some contexts to denote all the fields of a table row or composite value. It also has a special meaning when used as the argument of an aggregate function, namely that the aggregate does not require any explicit parameter.

- The period (`.`) is used in numeric constants, and to separate schema, table, and column names.

### Comments

A comment is a sequence of characters beginning with double dashes and extending to the end of the line, e.g.:

    -- This is a standard SQL comment

Alternatively, C-style block comments can be used:

    /* multiline comment
     * with nesting: /* nested block comment */
     */

where the comment begins with `/*` and extends to the matching occurrence of `*/`. These block comments nest, as specified in the SQL standard but unlike C, so that one can comment out larger blocks of code that might contain existing block comments.

A comment is removed from the input stream before further syntax analysis and is effectively replaced by whitespace.

### Operator Precedence

sql-precedence-table shows the precedence and associativity of the operators in PostgreSQL. Most operators have the same precedence and are left-associative. The precedence and associativity of the operators is hard-wired into the parser. Add parentheses if you want an expression with multiple operators to be parsed in some other way than what the precedence rules imply.

### Operator Precedence (highest to lowest)

Operator/Element | Associativity | Description
. | left | table/column name separator
:: | left | PostgreSQL-style typecast
[ ] | left | array element selection
+ - | right | unary plus, unary minus
COLLATE | left | collation selection
AT | left | `AT TIME ZONE`, `AT LOCAL`
^ | left | exponentiation
* / % | left | multiplication, division, modulo
+ - | left | addition, subtraction
(any other operator) | left | all other native and user-defined operators
BETWEEN IN LIKE ILIKE SIMILAR |  | range containment, set membership, string matching
< > = <= >= <> |  | comparison operators
IS ISNULL NOTNULL |  | `IS TRUE`, `IS FALSE`, `IS NULL`, `IS DISTINCT FROM`, etc.
NOT | right | logical negation
AND | left | logical conjunction
OR | left | logical disjunction

Note that the operator precedence rules also apply to user-defined operators that have the same names as the built-in operators mentioned above. For example, if you define a + operator for some custom data type it will have the same precedence as the built-in + operator, no matter what yours does.

When a schema-qualified operator name is used in the `OPERATOR` syntax, as for example in:

    SELECT 3 OPERATOR(pg_catalog.+) 4;

the `OPERATOR` construct is taken to have the default precedence shown in sql-precedence-table for any other operator. This is true no matter which specific operator appears inside `OPERATOR()`.

PostgreSQL versions before 9.5 used slightly different operator precedence rules. In particular, <= >= and <> used to be treated as generic operators; `IS` tests used to have higher priority; and `NOT BETWEEN` and related constructs acted inconsistently, being taken in some cases as having the precedence of `NOT` rather than `BETWEEN`. These rules were changed for better compliance with the SQL standard and to reduce confusion from inconsistent treatment of logically equivalent constructs. In most cases, these changes will result in no behavioral change, or perhaps in no such operator failures which can be resolved by adding parentheses. However there are corner cases in which a query might change behavior without any parsing error being reported.

## Value Expressions

Value expressions are used in a variety of contexts, such as in the target list of the `SELECT` command, as new column values in `INSERT` or `UPDATE`, or in search conditions in a number of commands. The result of a value expression is sometimes called a scalar, to distinguish it from the result of a table expression (which is a table). Value expressions are therefore also called scalar expressions (or even simply expressions). The expression syntax allows the calculation of values from primitive parts using arithmetic, logical, set, and other operations.

A value expression is one of the following:

- A constant or literal value

- A column reference

- A positional parameter reference, in the body of a function definition or prepared statement

- A subscripted expression

- A field selection expression

- An operator invocation

- A function call

- An aggregate expression

- A window function call

- A type cast

- A collation expression

- A scalar subquery

- An array constructor

- A row constructor

- Another value expression in parentheses (used to group subexpressions and override precedence
  )

In addition to this list, there are a number of constructs that can be classified as an expression but do not follow any general syntax rules. These generally have the semantics of a function or operator and are explained in the appropriate location in functions. An example is the `IS NULL` clause.

We have already discussed constants in sql-syntax-constants. The following sections discuss the remaining options.

### Column References

A column can be referenced in the form:

    correlation.columnname

`correlation` is the name of a table (possibly qualified with a schema name), or an alias for a table defined by means of a `FROM` clause. The correlation name and separating dot can be omitted if the column name is unique across all the tables being used in the current query. (See also queries.)

### Positional Parameters

A positional parameter reference is used to indicate a value that is supplied externally to an SQL statement. Parameters are used in SQL function definitions and in prepared queries. Some client libraries also support specifying data values separately from the SQL command string, in which case parameters are used to refer to the out-of-line data values. The form of a parameter reference is:

    $number

For example, consider the definition of a function, `dept`, as:

    CREATE FUNCTION dept(text) RETURNS dept
        AS $$ SELECT * FROM dept WHERE name = $1 $$
        LANGUAGE SQL;

Here the `$1` references the value of the first function argument whenever the function is invoked.

### Subscripts

If an expression yields a value of an array type, then a specific element of the array value can be extracted by writing

    expression[subscript]

or multiple adjacent elements (an array slice) can be extracted by writing

    expression[lower_subscript:upper_subscript]

(Here, the brackets `[ ]` are meant to appear literally.) Each `subscript` is itself an expression, which will be rounded to the nearest integer value.

In general the array `expression` must be parenthesized, but the parentheses can be omitted when the expression to be subscripted is just a column reference or positional parameter. Also, multiple subscripts can be concatenated when the original array is multidimensional. For example:

    mytable.arraycolumn[4]
    mytable.two_d_column[17][34]
    $1[10:42]
    (arrayfunction(a,b))[42]

The parentheses in the last example are required. See arrays for more about arrays.

### Field Selection

If an expression yields a value of a composite type (row type), then a specific field of the row can be extracted by writing

    expression.fieldname

In general the row `expression` must be parenthesized, but the parentheses can be omitted when the expression to be selected from is just a table reference or positional parameter. For example:

    mytable.mycolumn
    $1.somecolumn
    (rowfunction(a,b)).col3

(Thus, a qualified column reference is actually just a special case of the field selection syntax.) An important special case is extracting a field from a table column that is of a composite type:

    (compositecol).somefield
    (mytable.compositecol).somefield

The parentheses are required here to show that compositecol is a column name not a table name, or that mytable is a table name not a schema name in the second case.

You can ask for all fields of a composite value by writing `.*`:

    (compositecol).*

This notation behaves differently depending on context; see rowtypes-usage for details.

### Operator Invocations

There are two possible syntaxes for an operator invocation:

- `expression` `operator` `expression` (binary infix operator)
- `operator` `expression` (unary prefix operator)

where the `operator` token follows the syntax rules of sql-syntax-operators, or is one of the key words AND, OR, and NOT, or is a qualified operator name in the form:

    OPERATOR(schema.operatorname)

Which particular operators exist and whether they are unary or binary depends on what operators have been defined by the system or the user. functions describes the built-in operators.

### Function Calls

The syntax for a function call is the name of a function (possibly qualified with a schema name), followed by its argument list enclosed in parentheses:

    function_name (expression , expression ...  )

For example, the following computes the square root of 2:

    sqrt(2)

The list of built-in functions is in functions. Other functions can be added by the user.

When issuing queries in a database where some users mistrust other users, observe security precautions from typeconv-func when writing function calls.

The arguments can optionally have names attached. See sql-syntax-calling-funcs for details.

A function that takes a single argument of composite type can optionally be called using field-selection syntax, and conversely field selection can be written in functional style. That is, the notations `col(table)` and `table.col` are interchangeable. This behavior is not SQL-standard but is provided in PostgreSQL because it allows use of functions to emulate computed fields. For more information see rowtypes-usage.

### Aggregate Expressions

An aggregate expression represents the application of an aggregate function across the rows selected by a query. An aggregate function reduces multiple inputs to a single output value, such as the sum or average of the inputs. The syntax of an aggregate expression is one of the following:

    aggregate_name (expression [ , ... ] [ order_by_clause ] ) [ FILTER ( WHERE filter_clause ) ]
    aggregate_name (ALL expression [ , ... ] [ order_by_clause ] ) [ FILTER ( WHERE filter_clause ) ]
    aggregate_name (DISTINCT expression [ , ... ] [ order_by_clause ] ) [ FILTER ( WHERE filter_clause ) ]
    aggregate_name ( * ) [ FILTER ( WHERE filter_clause ) ]
    aggregate_name ( [ expression [ , ... ] ] ) WITHIN GROUP ( order_by_clause ) [ FILTER ( WHERE filter_clause ) ]

where `aggregate_name` is a previously defined aggregate (possibly qualified with a schema name) and `expression` is any value expression that does not itself contain an aggregate expression or a window function call. The optional `order_by_clause` and `filter_clause` are described below.

The first form of aggregate expression invokes the aggregate once for each input row. The second form is the same as the first, since `ALL` is the default. The third form invokes the aggregate once for each distinct value of the expression (or distinct set of values, for multiple expressions) found in the input rows. The fourth form invokes the aggregate once for each input row; since no particular input value is specified, it is generally only useful for the `count(*)` aggregate function. The last form is used with ordered-set aggregate functions, which are described below.

Most aggregate functions ignore null inputs, so that rows in which one or more of the expression(s) yield null are discarded. This can be assumed to be true, unless otherwise specified, for all built-in aggregates.

For example, `count(*)` yields the total number of input rows; `count(f1)` yields the number of input rows in which `f1` is non-null, since `count` ignores nulls; and `count(distinct f1)` yields the number of distinct non-null values of `f1`.

Ordinarily, the input rows are fed to the aggregate function in an unspecified order. In many cases this does not matter; for example, `min` produces the same result no matter what order it receives the inputs in. However, some aggregate functions (such as `array_agg` and `string_agg`) produce results that depend on the ordering of the input rows. When using such an aggregate, the optional `order_by_clause` can be used to specify the desired ordering. The `order_by_clause` has the same syntax as for a query-level `ORDER BY` clause, as described in queries-order, except that its expressions are always just expressions and cannot be output-column names or numbers. For example:

    WITH vals (v) AS ( VALUES (1),(3),(4),(3),(2) )
    SELECT array_agg(v ORDER BY v DESC) FROM vals;
      array_agg
    -------------
     {4,3,3,2,1}

Since `jsonb` only keeps the last matching key, ordering of its keys can be significant:

    WITH vals (k, v) AS ( VALUES ('key0','1'), ('key1','3'), ('key1','2') )
    SELECT jsonb_object_agg(k, v ORDER BY v) FROM vals;
          jsonb_object_agg
    ----------------------------
     {"key0": "1", "key1": "3"}

When dealing with multiple-argument aggregate functions, note that the `ORDER BY` clause goes after all the aggregate arguments. For example, write this:

    SELECT string_agg(a, ',' ORDER BY a) FROM table;

not this:

    SELECT string_agg(a ORDER BY a, ',') FROM table;  -- incorrect

The latter is syntactically valid, but it represents a call of a single-argument aggregate function with two `ORDER BY` keys (the second one being rather useless since it's a constant).

If `DISTINCT` is specified with an `order_by_clause`, `ORDER BY` expressions can only reference columns in the `DISTINCT` list. For example:

    WITH vals (v) AS ( VALUES (1),(3),(4),(3),(2) )
    SELECT array_agg(DISTINCT v ORDER BY v DESC) FROM vals;
     array_agg
    -----------
     {4,3,2,1}

Placing `ORDER BY` within the aggregate's regular argument list, as described so far, is used when ordering the input rows for general-purpose and statistical aggregates, for which ordering is optional. There is a subclass of aggregate functions called ordered-set aggregates for which an `order_by_clause` is required, usually because the aggregate's computation is only sensible in terms of a specific ordering of its input rows. Typical examples of ordered-set aggregates include rank and percentile calculations. For an ordered-set aggregate, the `order_by_clause` is written inside `WITHIN GROUP (...)`, as shown in the final syntax alternative above. The expressions in the `order_by_clause` are evaluated once per input row just like regular aggregate arguments, sorted as per the `order_by_clause`'s requirements, and fed to the aggregate function as input arguments. (This is unlike the case for a non-`WITHIN GROUP` `order_by_clause`, which is not treated as argument(s) to the aggregate function.) The argument expressions preceding `WITHIN GROUP`, if any, are called direct arguments to distinguish them from the aggregated arguments listed in the `order_by_clause`. Unlike regular aggregate arguments, direct arguments are evaluated only once per aggregate call, not once per input row. This means that they can contain variables only if those variables are grouped by `GROUP BY`; this restriction is the same as if the direct arguments were not inside an aggregate expression at all. Direct arguments are typically used for things like percentile fractions, which only make sense as a single value per aggregation calculation. The direct argument list can be empty; in this case, write just `()` not `(*)`. (PostgreSQL will actually accept either spelling, but only the first way conforms to the SQL standard.)

An example of an ordered-set aggregate call is:

    SELECT percentile_cont(0.5) WITHIN GROUP (ORDER BY income) FROM households;
     percentile_cont
    -----------------
               50489

which obtains the 50th percentile, or median, value of the income column from table households. Here, `0.5` is a direct argument; it would make no sense for the percentile fraction to be a value varying across rows.

If `FILTER` is specified, then only the input rows for which the `filter_clause` evaluates to true are fed to the aggregate function; other rows are discarded. For example:

    SELECT
        count(*) AS unfiltered,
        count(*) FILTER (WHERE i < 5) AS filtered
    FROM generate_series(1,10) AS s(i);
     unfiltered | filtered
    ------------+----------
             10 |        4
    (1 row)

The predefined aggregate functions are described in functions-aggregate. Other aggregate functions can be added by the user.

An aggregate expression can only appear in the result list or `HAVING` clause of a `SELECT` command. It is forbidden in other clauses, such as `WHERE`, because those clauses are logically evaluated before the results of aggregates are formed.

When an aggregate expression appears in a subquery (see sql-syntax-scalar-subqueries and functions-subquery), the aggregate is normally evaluated over the rows of the subquery. But an exception occurs if the aggregate's arguments (and `filter_clause` if any) contain only outer-level variables: the aggregate then belongs to the nearest such outer level, and is evaluated over the rows of that query. The aggregate expression as a whole is then an outer reference for the subquery it appears in, and acts as a constant over any one evaluation of that subquery. The restriction about appearing only in the result list or `HAVING` clause applies with respect to the query level that the aggregate belongs to.

### Window Function Calls

A window function call represents the application of an aggregate-like function over some portion of the rows selected by a query. Unlike non-window aggregate calls, this is not tied to grouping of the selected rows into a single output row — each row remains separate in the query output. However the window function has access to all the rows that would be part of the current row's group according to the grouping specification (`PARTITION BY` list) of the window function call. The syntax of a window function call is one of the following:

    function_name (expression , expression ... ) [ FILTER ( WHERE filter_clause ) ] OVER window_name
    function_name (expression , expression ... ) [ FILTER ( WHERE filter_clause ) ] OVER ( window_definition )
    function_name ( * ) [ FILTER ( WHERE filter_clause ) ] OVER window_name
    function_name ( * ) [ FILTER ( WHERE filter_clause ) ] OVER ( window_definition )

where `window_definition` has the syntax

    [ existing_window_name ]
    [ PARTITION BY expression [, ...] ]
    [ ORDER BY expression [ ASC | DESC | USING operator ] [ NULLS { FIRST | LAST } ] [, ...] ]
    [ frame_clause ]

The optional `frame_clause` can be one of

    { RANGE | ROWS | GROUPS } frame_start [ frame_exclusion ]
    { RANGE | ROWS | GROUPS } BETWEEN frame_start AND frame_end [ frame_exclusion ]

where `frame_start` and `frame_end` can be one of

    UNBOUNDED PRECEDING
    offset PRECEDING
    CURRENT ROW
    offset FOLLOWING
    UNBOUNDED FOLLOWING

and `frame_exclusion` can be one of

    EXCLUDE CURRENT ROW
    EXCLUDE GROUP
    EXCLUDE TIES
    EXCLUDE NO OTHERS

Here, `expression` represents any value expression that does not itself contain window function calls.

`window_name` is a reference to a named window specification defined in the query's `WINDOW` clause. Alternatively, a full `window_definition` can be given within parentheses, using the same syntax as for defining a named window in the `WINDOW` clause; see the sql-select reference page for details. It's worth pointing out that `OVER wname` is not exactly equivalent to `OVER (wname ...)`; the latter implies copying and modifying the window definition, and will be rejected if the referenced window specification includes a frame clause.

The `PARTITION BY` clause groups the rows of the query into partitions, which are processed separately by the window function. `PARTITION BY` works similarly to a query-level `GROUP BY` clause, except that its expressions are always just expressions and cannot be output-column names or numbers. Without `PARTITION BY`, all rows produced by the query are treated as a single partition. The `ORDER BY` clause determines the order in which the rows of a partition are processed by the window function. It works similarly to a query-level `ORDER BY` clause, but likewise cannot use output-column names or numbers. Without `ORDER BY`, rows are processed in an unspecified order.

The `frame_clause` specifies the set of rows constituting the window frame, which is a subset of the current partition, for those window functions that act on the frame instead of the whole partition. The set of rows in the frame can vary depending on which row is the current row. The frame can be specified in `RANGE`, `ROWS` or `GROUPS` mode; in each case, it runs from the `frame_start` to the `frame_end`. If `frame_end` is omitted, the end defaults to `CURRENT ROW`.

A `frame_start` of `UNBOUNDED PRECEDING` means that the frame starts with the first row of the partition, and similarly a `frame_end` of `UNBOUNDED FOLLOWING` means that the frame ends with the last row of the partition.

In `RANGE` or `GROUPS` mode, a `frame_start` of `CURRENT ROW` means the frame starts with the current row's first peer row (a row that the window's `ORDER BY` clause sorts as equivalent to the current row), while a `frame_end` of `CURRENT ROW` means the frame ends with the current row's last peer row. In `ROWS` mode, `CURRENT ROW` simply means the current row.

In the `offset` `PRECEDING` and `offset` `FOLLOWING` frame options, the `offset` must be an expression not containing any variables, aggregate functions, or window functions. The meaning of the `offset` depends on the frame mode:

- In `ROWS` mode, the `offset` must yield a non-null, non-negative integer, and the option means that the frame starts or ends the specified number of rows before or after the current row.

- In `GROUPS` mode, the `offset` again must yield a non-null, non-negative integer, and the option means that the frame starts or ends the specified number of peer groups before or after the current row's peer group, where a peer group is a set of rows that are equivalent in the `ORDER BY` ordering. (There must be an `ORDER BY` clause in the window definition to use `GROUPS` mode.)

- In `RANGE` mode, these options require that the `ORDER BY` clause specify exactly one column. The `offset` specifies the maximum difference between the value of that column in the current row and its value in preceding or following rows of the frame. The data type of the `offset` expression varies depending on the data type of the ordering column. For numeric ordering columns it is typically of the same type as the ordering column, but for datetime ordering columns it is an `interval`. For example, if the ordering column is of type `date` or `timestamp`, one could write `RANGE BETWEEN '1 day' PRECEDING AND '10 days' FOLLOWING`. The `offset` is still required to be non-null and non-negative, though the meaning of non-negative depends on its data type.

In any case, the distance to the end of the frame is limited by the distance to the end of the partition, so that for rows near the partition ends the frame might contain fewer rows than elsewhere.

Notice that in both `ROWS` and `GROUPS` mode, `0 PRECEDING` and `0 FOLLOWING` are equivalent to `CURRENT ROW`. This normally holds in `RANGE` mode as well, for an appropriate data-type-specific meaning of zero.

The `frame_exclusion` option allows rows around the current row to be excluded from the frame, even if they would be included according to the frame start and frame end options. `EXCLUDE CURRENT ROW` excludes the current row from the frame. `EXCLUDE GROUP` excludes the current row and its ordering peers from the frame. `EXCLUDE TIES` excludes any peers of the current row from the frame, but not the current row itself. `EXCLUDE NO OTHERS` simply specifies explicitly the default behavior of not excluding the current row or its peers.

The default framing option is `RANGE UNBOUNDED PRECEDING`, which is the same as `RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW`. With `ORDER BY`, this sets the frame to be all rows from the partition start up through the current row's last `ORDER BY` peer. Without `ORDER BY`, this means all rows of the partition are included in the window frame, since all rows become peers of the current row.

Restrictions are that `frame_start` cannot be `UNBOUNDED FOLLOWING`, `frame_end` cannot be `UNBOUNDED PRECEDING`, and the `frame_end` choice cannot appear earlier in the above list of `frame_start` and `frame_end` options than the `frame_start` choice does — for example `RANGE BETWEEN CURRENT ROW AND `offset` PRECEDING` is not allowed. But, for example, `ROWS BETWEEN 7 PRECEDING AND 8 PRECEDING` is allowed, even though it would never select any rows.

If `FILTER` is specified, then only the input rows for which the `filter_clause` evaluates to true are fed to the window function; other rows are discarded. Only window functions that are aggregates accept a `FILTER` clause.

The built-in window functions are described in functions-window-table. Other window functions can be added by the user. Also, any built-in or user-defined general-purpose or statistical aggregate can be used as a window function. (Ordered-set and hypothetical-set aggregates cannot presently be used as window functions.)

The syntaxes using `*` are used for calling parameter-less aggregate functions as window functions, for example `count(*) OVER (PARTITION BY x ORDER BY y)`. The asterisk (`*`) is customarily not used for window-specific functions. Window-specific functions do not allow `DISTINCT` or `ORDER BY` to be used within the function argument list.

Window function calls are permitted only in the `SELECT` list and the `ORDER BY` clause of the query.

More information about window functions can be found in tutorial-window, functions-window, and queries-window.

### Type Casts

A type cast specifies a conversion from one data type to another. PostgreSQL accepts two equivalent syntaxes for type casts:

    CAST ( expression AS type )
    expression::type

The `CAST` syntax conforms to SQL; the syntax with `::` is historical PostgreSQL usage.

When a cast is applied to a value expression of a known type, it represents a run-time type conversion. The cast will succeed only if a suitable type conversion operation has been defined. Notice that this is subtly different from the use of casts with constants, as shown in sql-syntax-constants-generic. A cast applied to an unadorned string literal represents the initial assignment of a type to a literal constant value, and so it will succeed for any type (if the contents of the string literal are acceptable input syntax for the data type).

An explicit type cast can usually be omitted if there is no ambiguity as to the type that a value expression must produce (for example, when it is assigned to a table column); the system will automatically apply a type cast in such cases. However, automatic casting is only done for casts that are marked OK to apply implicitly in the system catalogs. Other casts must be invoked with explicit casting syntax. This restriction is intended to prevent surprising conversions from being applied silently.

It is also possible to specify a type cast using a function-like syntax:

    typename ( expression )

However, this only works for types whose names are also valid as function names. For example, `double precision` cannot be used this way, but the equivalent `float8` can. Also, the names `interval`, `time`, and `timestamp` can only be used in this fashion if they are double-quoted, because of syntactic conflicts. Therefore, the use of the function-like cast syntax leads to inconsistencies and should probably be avoided.

The function-like syntax is in fact just a function call. When one of the two standard cast syntaxes is used to do a run-time conversion, it will internally invoke a registered function to perform the conversion. By convention, these conversion functions have the same name as their output type, and thus the function-like syntax is nothing more than a direct invocation of the underlying conversion function. Obviously, this is not something that a portable application should rely on. For further details see sql-createcast.

### Collation Expressions

The `COLLATE` clause overrides the collation of an expression. It is appended to the expression it applies to:

    expr COLLATE collation

where `collation` is a possibly schema-qualified identifier. The `COLLATE` clause binds tighter than operators; parentheses can be used when necessary.

If no collation is explicitly specified, the database system either derives a collation from the columns involved in the expression, or it defaults to the default collation of the database if no column is involved in the expression.

The two common uses of the `COLLATE` clause are overriding the sort order in an `ORDER BY` clause, for example:

    SELECT a, b, c FROM tbl WHERE ... ORDER BY a COLLATE "C";

and overriding the collation of a function or operator call that has locale-sensitive results, for example:

    SELECT * FROM tbl WHERE a > 'foo' COLLATE "C";

Note that in the latter case the `COLLATE` clause is attached to an input argument of the operator we wish to affect. It doesn't matter which argument of the operator or function call the `COLLATE` clause is attached to, because the collation that is applied by the operator or function is derived by considering all arguments, and an explicit `COLLATE` clause will override the collations of all other arguments. (Attaching non-matching `COLLATE` clauses to more than one argument, however, is an error. For more details see collation.) Thus, this gives the same result as the previous example:

    SELECT * FROM tbl WHERE a COLLATE "C" > 'foo';

But this is an error:

    SELECT * FROM tbl WHERE (a > 'foo') COLLATE "C";

because it attempts to apply a collation to the result of the `>` operator, which is of the non-collatable data type `boolean`.

### Scalar Subqueries

A scalar subquery is an ordinary `SELECT` query in parentheses that returns exactly one row with one column. (See queries for information about writing queries.) The `SELECT` query is executed and the single returned value is used in the surrounding value expression. It is an error to use a query that returns more than one row or more than one column as a scalar subquery. (But if, during a particular execution, the subquery returns no rows, there is no error; the scalar result is taken to be null.) The subquery can refer to variables from the surrounding query, which will act as constants during any one evaluation of the subquery. See also functions-subquery for other expressions involving subqueries.

For example, the following finds the largest city population in each state:

    SELECT name, (SELECT max(pop) FROM cities WHERE cities.state = states.name)
        FROM states;

### Array Constructors

An array constructor is an expression that builds an array value using values for its member elements. A simple array constructor consists of the key word `ARRAY`, a left square bracket `[`, a list of expressions (separated by commas) for the array element values, and finally a right square bracket `]`. For example:

    SELECT ARRAY[1,2,3+4];
      array
    ---------
     {1,2,7}
    (1 row)

By default, the array element type is the common type of the member expressions, determined using the same rules as for `UNION` or `CASE` constructs (see typeconv-union-case). You can override this by explicitly casting the array constructor to the desired type, for example:

    SELECT ARRAY[1,2,22.7]::integer[];
      array
    ----------
     {1,2,23}
    (1 row)

This has the same effect as casting each expression to the array element type individually. For more on casting, see sql-syntax-type-casts.

Multidimensional array values can be built by nesting array constructors. In the inner constructors, the key word `ARRAY` can be omitted. For example, these produce the same result:

    SELECT ARRAY[ARRAY[1,2], ARRAY[3,4]];
         array
    ---------------
     {{1,2},{3,4}}
    (1 row)

    SELECT ARRAY[[1,2],[3,4]];
         array
    ---------------
     {{1,2},{3,4}}
    (1 row)

Since multidimensional arrays must be rectangular, inner constructors at the same level must produce sub-arrays of identical dimensions. Any cast applied to the outer `ARRAY` constructor propagates automatically to all the inner constructors.

Multidimensional array constructor elements can be anything yielding an array of the proper kind, not only a sub-`ARRAY` construct. For example:

    CREATE TABLE arr(f1 int[], f2 int[]);

    INSERT INTO arr VALUES (ARRAY[[1,2],[3,4]], ARRAY[[5,6],[7,8]]);

    SELECT ARRAY[f1, f2, '{{9,10},{11,12}}'::int[]] FROM arr;
                         array
    ------------------------------------------------
     {{{1,2},{3,4}},{{5,6},{7,8}},{{9,10},{11,12}}}
    (1 row)

You can construct an empty array, but since it's impossible to have an array with no type, you must explicitly cast your empty array to the desired type. For example:

    SELECT ARRAY[]::integer[];
     array
    -------
     {}
    (1 row)

It is also possible to construct an array from the results of a subquery. In this form, the array constructor is written with the key word `ARRAY` followed by a parenthesized (not bracketed) subquery. For example:

    SELECT ARRAY(SELECT oid FROM pg_proc WHERE proname LIKE 'bytea%');
                                  array
    ------------------------------------------------------------------
     {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31,2412}
    (1 row)

    SELECT ARRAY(SELECT ARRAY[i, i*2] FROM generate_series(1,5) AS a(i));
                  array
    ----------------------------------
     {{1,2},{2,4},{3,6},{4,8},{5,10}}
    (1 row)

The subquery must return a single column. If the subquery's output column is of a non-array type, the resulting one-dimensional array will have an element for each row in the subquery result, with an element type matching that of the subquery's output column. If the subquery's output column is of an array type, the result will be an array of the same type but one higher dimension; in this case all the subquery rows must yield arrays of identical dimensionality, else the result would not be rectangular.

The subscripts of an array value built with `ARRAY` always begin with one. For more information about arrays, see arrays.

### Row Constructors

A row constructor is an expression that builds a row value (also called a composite value) using values for its member fields. A row constructor consists of the key word `ROW`, a left parenthesis, zero or more expressions (separated by commas) for the row field values, and finally a right parenthesis. For example:

    SELECT ROW(1,2.5,'this is a test');

The key word `ROW` is optional when there is more than one expression in the list.

A row constructor can include the syntax `rowvalue``.*`, which will be expanded to a list of the elements of the row value, just as occurs when the `.*` syntax is used at the top level of a `SELECT` list (see rowtypes-usage). For example, if table `t` has columns `f1` and `f2`, these are the same:

    SELECT ROW(t.*, 42) FROM t;
    SELECT ROW(t.f1, t.f2, 42) FROM t;

Before PostgreSQL 8.2, the `.*` syntax was not expanded in row constructors, so that writing `ROW(t.*, 42)` created a two-field row whose first field was another row value. The new behavior is usually more useful. If you need the old behavior of nested row values, write the inner row value without `.*`, for instance `ROW(t, 42)`.

By default, the value created by a `ROW` expression is of an anonymous record type. If necessary, it can be cast to a named composite type — either the row type of a table, or a composite type created with `CREATE TYPE AS`. An explicit cast might be needed to avoid ambiguity. For example:

    CREATE TABLE mytable(f1 int, f2 float, f3 text);

    CREATE FUNCTION getf1(mytable) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;

    -- No cast needed since only one getf1() exists
    SELECT getf1(ROW(1,2.5,'this is a test'));
     getf1
    -------
         1
    (1 row)

    CREATE TYPE myrowtype AS (f1 int, f2 text, f3 numeric);

    CREATE FUNCTION getf1(myrowtype) RETURNS int AS 'SELECT $1.f1' LANGUAGE SQL;

    -- Now we need a cast to indicate which function to call:
    SELECT getf1(ROW(1,2.5,'this is a test'));
    ERROR:  function getf1(record) is not unique

    SELECT getf1(ROW(1,2.5,'this is a test')::mytable);
     getf1
    -------
         1
    (1 row)

    SELECT getf1(CAST(ROW(11,'this is a test',2.5) AS myrowtype));
     getf1
    -------
        11
    (1 row)

Row constructors can be used to build composite values to be stored in a composite-type table column, or to be passed to a function that accepts a composite parameter. Also, it is possible to test rows using the standard comparison operators as described in functions-comparison, to compare one row against another as described in functions-comparisons, and to use them in connection with subqueries, as discussed in functions-subquery,

### Expression Evaluation Rules

The order of evaluation of subexpressions is not defined. In particular, the inputs of an operator or function are not necessarily evaluated left-to-right or in any other fixed order.

Furthermore, if the result of an expression can be determined by evaluating only some parts of it, then other subexpressions might not be evaluated at all. For instance, if one wrote:

    SELECT true OR somefunc();

then `somefunc()` would (probably) not be called at all. The same would be the case if one wrote:

    SELECT somefunc() OR true;

Note that this is not the same as the left-to-right short-circuiting of Boolean operators that is found in some programming languages.

As a consequence, it is unwise to use functions with side effects as part of complex expressions. It is particularly dangerous to rely on side effects or evaluation order in `WHERE` and `HAVING` clauses, since those clauses are extensively reprocessed as part of developing an execution plan. Boolean expressions (`AND`/`OR`/`NOT` combinations) in those clauses can be reorganized in any manner allowed by the laws of Boolean algebra.

When it is essential to force evaluation order, a `CASE` construct (see functions-conditional) can be used. For example, this is an untrustworthy way of trying to avoid division by zero in a `WHERE` clause:

    SELECT ... WHERE x > 0 AND y/x > 1.5;

But this is safe:

    SELECT ... WHERE CASE WHEN x > 0 THEN y/x > 1.5 ELSE false END;

A `CASE` construct used in this fashion will defeat optimization attempts, so it should only be done when necessary. (In this particular example, it would be better to sidestep the problem by writing `y > 1.5*x` instead.)

`CASE` is not a cure-all for such issues, however. One limitation of the technique illustrated above is that it does not prevent early evaluation of constant subexpressions. As described in xfunc-volatility, functions and operators marked `IMMUTABLE` can be evaluated when the query is planned rather than when it is executed. Thus for example

    SELECT CASE WHEN x > 0 THEN x ELSE 1/0 END FROM tab;

is likely to result in a division-by-zero failure due to the planner trying to simplify the constant subexpression, even if every row in the table has `x > 0` so that the `ELSE` arm would never be entered at run time.

While that particular example might seem silly, related cases that don't obviously involve constants can occur in queries executed within functions, since the values of function arguments and local variables can be inserted into queries as constants for planning purposes. Within PL/pgSQL functions, for example, using an `IF`-`THEN`-`ELSE` statement to protect a risky computation is much safer than just nesting it in a `CASE` expression.

Another limitation of the same kind is that a `CASE` cannot prevent evaluation of an aggregate expression contained within it, because aggregate expressions are computed before other expressions in a `SELECT` list or `HAVING` clause are considered. For example, the following query can cause a division-by-zero error despite seemingly having protected against it:

    SELECT CASE WHEN min(employees) > 0
                THEN avg(expenses / employees)
           END
        FROM departments;

The `min()` and `avg()` aggregates are computed concurrently over all the input rows, so if any row has employees equal to zero, the division-by-zero error will occur before there is any opportunity to test the result of `min()`. Instead, use a `WHERE` or `FILTER` clause to prevent problematic input rows from reaching an aggregate function in the first place.

## Calling Functions

PostgreSQL allows functions that have named parameters to be called using either positional or named notation. Named notation is especially useful for functions that have a large number of parameters, since it makes the associations between parameters and actual arguments more explicit and reliable. In positional notation, a function call is written with its argument values in the same order as they are defined in the function declaration. In named notation, the arguments are matched to the function parameters by name and can be written in any order. For each notation, also consider the effect of function argument types, documented in typeconv-func.

In either notation, parameters that have default values given in the function declaration need not be written in the call at all. But this is particularly useful in named notation, since any combination of parameters can be omitted; while in positional notation parameters can only be omitted from right to left.

PostgreSQL also supports mixed notation, which combines positional and named notation. In this case, positional parameters are written first and named parameters appear after them.

The following examples will illustrate the usage of all three notations, using the following function definition:

    CREATE FUNCTION concat_lower_or_upper(a text, b text, uppercase boolean DEFAULT false)
    RETURNS text
    AS
    $$
     SELECT CASE
            WHEN $3 THEN UPPER($1 || ' ' || $2)
            ELSE LOWER($1 || ' ' || $2)
            END;
    $$
    LANGUAGE SQL IMMUTABLE STRICT;

Function `concat_lower_or_upper` has two mandatory parameters, `a` and `b`. Additionally there is one optional parameter `uppercase` which defaults to `false`. The `a` and `b` inputs will be concatenated, and forced to either upper or lower case depending on the `uppercase` parameter. The remaining details of this function definition are not important here (see extend for more information).

### Using Positional Notation

Positional notation is the traditional mechanism for passing arguments to functions in PostgreSQL. An example is:

    SELECT concat_lower_or_upper('Hello', 'World', true);
     concat_lower_or_upper
    -----------------------
     HELLO WORLD
    (1 row)

All arguments are specified in order. The result is upper case since `uppercase` is specified as `true`. Another example is:

    SELECT concat_lower_or_upper('Hello', 'World');
     concat_lower_or_upper
    -----------------------
     hello world
    (1 row)

Here, the `uppercase` parameter is omitted, so it receives its default value of `false`, resulting in lower case output. In positional notation, arguments can be omitted from right to left so long as they have defaults.

### Using Named Notation

In named notation, each argument's name is specified using `=>` to separate it from the argument expression. For example:

    SELECT concat_lower_or_upper(a => 'Hello', b => 'World');
     concat_lower_or_upper
    -----------------------
     hello world
    (1 row)

Again, the argument `uppercase` was omitted so it is set to `false` implicitly. One advantage of using named notation is that the arguments may be specified in any order, for example:

    SELECT concat_lower_or_upper(a => 'Hello', b => 'World', uppercase => true);
     concat_lower_or_upper
    -----------------------
     HELLO WORLD
    (1 row)

    SELECT concat_lower_or_upper(a => 'Hello', uppercase => true, b => 'World');
     concat_lower_or_upper
    -----------------------
     HELLO WORLD
    (1 row)

An older syntax based on ":=" is supported for backward compatibility:

    SELECT concat_lower_or_upper(a := 'Hello', uppercase := true, b := 'World');
     concat_lower_or_upper
    -----------------------
     HELLO WORLD
    (1 row)

### Using Mixed Notation

The mixed notation combines positional and named notation. However, as already mentioned, named arguments cannot precede positional arguments. For example:

    SELECT concat_lower_or_upper('Hello', 'World', uppercase => true);
     concat_lower_or_upper
    -----------------------
     HELLO WORLD
    (1 row)

In the above query, the arguments `a` and `b` are specified positionally, while `uppercase` is specified by name. In this example, that adds little except documentation. With a more complex function having numerous parameters that have default values, named or mixed notation can save a great deal of writing and reduce chances for error.

Named and mixed call notations currently cannot be used when calling an aggregate function (but they do work when an aggregate function is used as a window function).