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;