4. Elements of OpenSQL Statements : Functions : Aggregate Functions : Restrictions on the Use of Aggregate Functions
 
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Restrictions on the Use of Aggregate Functions
The following restrictions apply to the use of aggregate functions:
Aggregate functions cannot be nested.
Aggregate functions can only be used in select or having clauses.
If a select or having clause contains an aggregate function, columns not specified in the aggregate must be specified in the group by clause. For example:
select dept, avg(emp_age)
from employee
    group by dept;
The above select statement specifies two columns, dept and emp_age, but only emp_age is referenced by the aggregate function, avg. The dept column is specified in the group by clause.