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:
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 8. An example is the IS NULL
clause.
We have already discussed constants in Section 3.1.2. 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 6.)
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 3.1.3, 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 8
describes the built-in operators.
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 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 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 Section 8.11.
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 3.2.4 and
Section 8.12), the aggregate is normally
evaluated over the rows of the subquery. But an exception occurs
if the aggregate's argument contains 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.
A scalar subquery is an ordinary
SELECT query in parentheses that returns exactly one
row with one column. (See Chapter 6 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 8.12 for other expressions involving subqueries.
For example, the following finds the highest paid employee in each department:
SELECT ename, (SELECT max(sal) FROM emp,dept WHERE emp.deptno = dept.deptno) AS dept_max_salary
FROM emp;