--- avg

The `avg' aggregate function calculates the average value of an
   expression composed of constants and data base fields connected
   by the standard arithmetic operators: +, -, *, /.

Aggregate functions are only valid when used in `select' or `having'
   clauses.

EXAMPLE: 

          sql> select dept_no, job, avg(salary)

          sql> select job, avg (salary + commission)
