Aggregate functions allow you to calculate values based upon all data in an attribute of a table. The SQL aggregate functions are:  Max, Min, Avg, Sum, Count, StdDev, Variance.  Note that AVG and SUM work only with numeric values and both exclude NULL values from the calculations.

Example 1

How many students are there?
SELECT COUNT(*) FROM STUDENT;

NOTE: COUNT can be used in two ways.  COUNT(*) is used to count the number of tuples that satisfy a query.  COUNT with DISTINCT is used to count the number of unique values in a named column.

Example 2

Find the number of departments that have faculty in them.
SELECT COUNT(DISTINCT DEPT) FROM FACULTY;

Example 3

Find the average number of credits for students who major in math.
SELECT AVG(CREDITS) FROM STUDENT WHERE MAJOR = ‘Math’;