Aggregate Functions:

 They are used to compute a single value from a set  of attribute values of a column:

   count :

            how many rows(tuples) are stored in the relation EMP

select count(*) from EMP;

            how many different job title  are stored in the relation EMP

select count(distinct job) from EMP;

max & min:

           select min(SAL),max(SAL) from EMP

           select max(SAL)- min(SAL)  from EMP

sum:

         select sum(SAL) from EMP

avg: