SUM( expression )

An aggregate expression calculates a single value from a range of rows.

An aggregate expression is one in which either an aggregate function is used, or in which one or more of the operands is an aggregate expression.

When a SELECT statement does not have a GROUP BY clause, the expressions in the select-list must either contain all aggregate expressions or no aggregate expressions. When a SELECT statement does have a GROUP BY clause, any non-aggregate expression in the select-list must appear in the GROUP BY list.

For example, the following query computes the total payroll for employees in the employee table. In this query, SUM( salary ) is an aggregate expression:

SELECT SUM( salary )
FROM employee;