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 type cast.
A scalar subquery.
An array constructor.
A row constructor.
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 Chapter 9, Functions and Operators. An example is the IS NULL
clause.
We have already discussed constants in Section 4.1.2, “Constants”. The following sections discuss the remaining options.
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, or one of
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 Chapter 7, Queries.)
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.
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 must yield an integer value.
In general the array expression
must be
parenthesized, but the parentheses may 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 Section 8.10, “Arrays” for more about arrays.
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 may 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.)
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 Section 4.1.3, “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. Chapter 9, Functions and Operators describes the built-in operators.
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 Chapter 9, Functions and Operators. 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
[ , ... ] )aggregate_name
(ALLexpression
[ , ... ] )aggregate_name
(DISTINCTexpression
[ , ... ] )aggregate_name
( * )
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.
The first form of aggregate expression invokes the aggregate
across all input rows for which the given expression(s) yield
non-null values. (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 expressions 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 9.15, “Aggregate Functions”. Other aggregate functions may be added by the user.
An aggregate expression may 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
Section 4.2.9, “Scalar Subqueries” and
Section 9.16, “Subquery Expressions”), the aggregate is normally
evaluated over the rows of the subquery. But an exception occurs
if the aggregate's arguments 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.
PostgreSQL currently does not support
DISTINCT
with more than one input expression.
A type cast specifies a conversion from one data type to another. PostgreSQL accepts two equivalent syntaxes for type casts:
CAST (expression
AStype
)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 Section 4.1.2.5, “Constants of Other Types”. 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, 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.)
A scalar subquery is an ordinary
SELECT
query in parentheses that returns exactly one
row with one column. (See Chapter 7, 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 Section 9.16, “Subquery Expressions” 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;
An array constructor is an expression that builds an
array value from values for its member elements. A simple array
constructor
consists of the key word ARRAY
, a left square bracket
[
, one or more 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)
The array element type is the common type of the member expressions,
determined using the same rules as for UNION
or
CASE
constructs (see Section 10.5, “UNION
, CASE
, and Related Constructs”).
Multidimensional array values can be built by nesting array
constructors.
In the inner constructors, the key word ARRAY
may
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.
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)
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%'); ?column? ------------------------------------------------------------- {2011,1954,1948,1952,1951,1244,1950,2005,1949,1953,2006,31} (1 row)
The subquery must return a single column. 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.
The subscripts of an array value built with ARRAY
always begin with one. For more information about arrays, see
Section 8.10, “Arrays”.
A row constructor is an expression that builds a row value (also
called a composite value) from 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. 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, 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 may 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 compare two row values or test a row with
IS NULL
or IS NOT NULL
, for example
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same'); SELECT ROW(table.*) IS NULL FROM table; -- detect all-null rows
For more detail see Section 9.17, “Row and Array Comparisons”. Row constructors can also be used in connection with subqueries, as discussed in Section 9.16, “Subquery Expressions”.
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 Section 9.13, “Conditional Expressions”) 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. (In this particular
example, it would doubtless be best to sidestep the problem by writing
y > 1.5*x
instead.)