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 Section 1.1.2.

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.

(

)*expression*Parentheses are 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 Chapter 4. An example is the `IS NULL`
clause.

We have already discussed constants in Section 1.1.2. The following sections discuss the remaining options.

A column can be referenced in the form:

.correlation`['columnname`]'subscript

` correlation` is either the name of a
table, an alias for a table defined by means of a FROM clause, or
the key words

A positional parameter reference is used to indicate a parameter in an SQL function. Typically this is used in SQL function definition statements. The form of a parameter 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.

There are three possible syntaxes for an operator invocation:

expressionoperator (binary infix operator)expression |

operator (unary prefix operator)expression |

expression (unary postfix operator)operator |

The syntax for a function call is the name of a function (which is subject to the syntax rules for identifiers of Section 1.1.1), 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 Chapter 4. Other functions may be added by the user.

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 |

(ALL aggregate_name)expression |

(DISTINCT aggregate_name)expression |

( * )aggregate_name |

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 NULLs 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 Section 4.14. Other aggregate functions may be added by the user.

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

CAST (ASexpression)type::expressiontype

The `CAST` syntax conforms to SQL92; 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 Section 1.1.2.5. 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 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.

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 parser conflicts. Therefore, the use of
the function-like cast syntax leads to inconsistencies and should
probably be avoided in new applications.

A scalar subquery is an ordinary
**SELECT** in parentheses that returns exactly one
row with one column. 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 Section 4.15.

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;