The DISTINCT option specifies that duplicate values are not to be used when an expression is processed.
The following SELECT returns the number of unique job titles in a table.
SELECT COUNT(DISTINCT JobTitle) FROM Employee;
A query can have multiple aggregate functions that use DISTINCT with the same expression, as shown by the following example.
SELECT SUM(DISTINCT x), AVG(DISTINCT x) FROM XTable;
A query can also have multiple aggregate functions that use DISTINCT with different expressions, for example:
SELECT SUM(DISTINCT x), SUM(DISTINCT y) FROM XYTable;