Set Functions 
Set functions are aggregate functions (they return one value for groups of rows and therefore imply grouping of tables).
Syntax
Syntax
<set function> ::= COUNT '(' '*' ')' | COUNT '(' DISTINCT <value expression> ')' | <general set function>. <general set function> ::= ( SUM | AVG | MIN | MAX ) '(' ( DISTINCT | ALL )? <value expression> ')'.
End of the code.
The return type of set functions follow these rules:
COUNT: The return type is exact numeric with scale 0.
AVG: The return type is approximate numeric.
MIN, MAX, SUM: The return type is the type of the argument.
Examples
Syntax
SELECT AVG(salary) FROM employees
End of the code.
Average. This query determines the average salary of all employees.
Syntax
SELECT COUNT(*) FROM employees
End of the code.
COUNT(*). This query determines the number of employees.
Syntax
SELECT COUNT(DISTINCT employee_name) FROM employees
End of the code.
Average. This query determines the number of different names of the employees.