Scalar functions
A scalar function takes input argument(s) and returns a single value result. A scalar function can be used wherever an expression can be used.
The restrictions on the use of aggregate functions do not apply to scalar functions, because a scalar function is applied to single parameter values rather than to sets of values. The argument of a scalar function can be a function. However, the restrictions that apply to the use of expressions and aggregate functions also apply when an expression or aggregate function is used within a scalar function. For example, the argument of a scalar function can be an aggregate function only if an aggregate function is allowed in the context in which the scalar function is used.
Example
The result of the following SELECT statement has as many rows as there are employees in department D01:
SELECT EMPNO, LASTNAME, YEAR(CURRENT DATE - BIRTHDATE)
FROM EMPLOYEE
WHERE WORKDEPT = 'D01'