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 subscripted expression.
An operator invocation.
An aggregate expression.
A scalar subquery.
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) |
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. 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 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 WHERE emp.deptno = dept.deptno) AS dept_max_salary FROM emp;