# 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 the 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 there are 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).

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

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).

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 the &cite-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, digits (`0`-`9`), or underscores, although the SQL standard will not define a key word that contains digits or starts or ends with an underscore.

The system uses no more than NAMEDATALEN-1 characters 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 (but at the time PostgreSQL is built, NAMEDATALEN can be changed in `src/include/postgres_ext.h`).

Identifier and key word names 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 other than a double quote itself. 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.

### 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. The implicit constants are described below; explicit constants are discussed afterwards.

#### String Constants

A string constant in SQL is an arbitrary sequence of characters bounded by single quotes ('), e.g., `'This is a string'`. SQL allows single quotes to be embedded in strings by typing two adjacent single quotes (e.g., `'Dianne''s horse'`). In PostgreSQL single quotes may alternatively be escaped with a backslash (\, e.g., `'Dianne\'s horse'`).

C-style backslash escapes are also available: `\b` is a backspace, `\f` is a form feed, `\n` is a newline, `\r` is a carriage return, `\t` is a tab, and `\`xxx``, where `xxx` is an octal number, is the character with the corresponding ASCII code. Any other character following a backslash is taken literally. Thus, to include a backslash in a string constant, type two backslashes.

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

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 in 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.)

#### Bit-String Constants

Bit-string constants look like 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.

#### 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 may not be any spaces or other characters embedded in the constant. 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

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

#### 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'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 may be omitted if there is no ambiguity as to the type the constant must be (for example, when it is passed as an argument to a non-overloaded function), in which case it is automatically coerced.

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

    typename ( 'string' )

but not all type names may 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. But the form `type` '`string`' can only be used to specify the type of a literal constant. Another restriction on `type` '`string`' is that it does not work for array types; use `::` or `CAST()` to specify the type of an array constant.

#### Array constants

The general format of an array constant is the following:

    '{ val1 delim val2 delim ... }'

where `delim` is the delimiter character for the type, as recorded in its `pg_type` entry. (For all built-in types, this is the comma character `,`.) Each `val` is either a constant of the array element type, or a subarray. An example of an array constant is

    '{{1,2,3},{4,5,6},{7,8,9}}'

This constant is a two-dimensional, 3-by-3 array consisting of three subarrays of integers.

Individual array elements can be placed between double-quote marks (`"`) to avoid ambiguity problems with respect to whitespace. Without quote marks, the array-value parser will skip leading whitespace.

(Array constants are actually only a special case of the generic type constants discussed in the previous section. The constant is initially treated as a string and passed to the array input conversion routine. An explicit type specification might be necessary.)

### Operators

An operator 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:

- `$` (dollar) cannot be a single-character operator, although it can be part of a multiple-character operator name.

- `--` 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 left unary 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 the positional parameters in the body of a function definition. In other contexts the dollar sign may be part of an operator name.

- 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 (`*`) has a special meaning when used in the `SELECT` command or with the `COUNT` aggregate function.

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

### Comments

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

    -- This is a standard SQL92 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 SQL99 but unlike C, so that one can comment out larger blocks of code that may contain existing block comments.

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

### Lexical 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. This may lead to non-intuitive behavior; for example the Boolean operators `<` and `>` have a different precedence than the Boolean operators `<=` and `>=`. Also, you will sometimes need to add parentheses when using combinations of binary and unary operators. For instance

    SELECT 5 ! - 6;

will be parsed as

    SELECT 5 ! (- 6);

because the parser has no idea -- until it is too late -- that ! is defined as a postfix operator, not an infix one. To get the desired behavior in this case, you must write

    SELECT (5 !) - 6;

This is the price one pays for extensibility.

### Operator Precedence (decreasing)

Operator/Element | Associativity | Description
. | left | table/column name separator
:: | left | PostgreSQL-style typecast
[ ] | left | array element selection
- | right | unary minus
^ | left | exponentiation
* / % | left | multiplication, division, modulo
+ - | left | addition, subtraction
IS |  | `IS TRUE`, `IS FALSE`, `IS UNKNOWN`, `IS NULL`
ISNULL |  | test for null
NOTNULL |  | test for not null
(any other) | left | all other native and user-defined operators
IN |  | set membership
BETWEEN |  | containment
OVERLAPS |  | time interval overlap
LIKE ILIKE SIMILAR |  | string pattern matching
< > |  | less than, greater than
= | right | equality, assignment
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 name appears inside `OPERATOR()`.

## 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; see sql-syntax-constants.

- A column reference.

- A positional parameter reference, in the body of a function declaration.

- An operator invocation.

- A function call.

- An aggregate expression.

- A type cast.

- A scalar subquery.

- Another value expression in parentheses, useful 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

or

    correlation.columnname[subscript]

(Here, the brackets `[ ]` are meant to appear literally.)

`correlation` is the name of a table (possibly qualified), or an alias for a table defined by means of a `FROM` clause, or the key words `NEW` or `OLD`. (`NEW` and `OLD` can only appear in rewrite rules, while other correlation names can be used in any SQL statement.) The correlation name and separating dot may be omitted if the column name is unique across all the tables being used in the current query. (See also queries.)

If `column` is of an array type, then the optional `subscript` selects a specific element or elements in the array. If no subscript is provided, then the whole array is selected. (See arrays for more about arrays.)

### Positional Parameters

A positional parameter reference is used to indicate a parameter that is supplied externally to an SQL statement. Parameters are used in SQL function definitions and in prepared queries. 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` will be replaced by the first function argument when the function is invoked.

### Operator Invocations

There are three possible syntaxes for an operator invocation:

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

where the `operator` token follows the syntax rules of sql-syntax-operators, or is one of the keywords AND, OR, and NOT, or is a qualified operator name

    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 (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 may be added by the user.

### 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)
    aggregate_name (ALL expression)
    aggregate_name (DISTINCT expression)
    aggregate_name ( * )

where `aggregate_name` is a previously defined aggregate (possibly a qualified name), and `expression` is any value expression that does not itself contain an aggregate expression.

The first form of aggregate expression invokes the aggregate across all input rows for which the given expression yields a non-null value. (Actually, it is up to the aggregate function whether to ignore null values or not --- but all the standard ones do.) The second form is the same as the first, since `ALL` is the default. The third form invokes the aggregate for all distinct non-null values of the expression found in the input rows. The last form invokes the aggregate once for each input row regardless of null or non-null values; since no particular input value is specified, it is generally only useful for the `count()` aggregate function.

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

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

### 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 function is available. 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 may 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` can't 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 in new applications. (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, but this is not something that a portable application should rely on.)

### 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 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;

### Expression Evaluation

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 may 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) may 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.