If you were to use

    SELECT E_NUM
    FROM SALES;

You would get

    E_NUM
    ------
    101
    102
   101
    ------

In this case, Employee number 101 has been reported twice even though once is enough. In this case, you would use the DISTINCT operator to filter out duplicates.

    SELECT DISTINCT E_NUM
    FROM SALES;

You would get

    E_NUM
    ------
    101
    102
    ------

Another common usage of distinct is to use it with a COUNT such as:

    SELECT num_successful_salesmen  =
           COUNT (DISTINCT E_NUM)
    FROM SALES;

which would produce:

    num_successful_salesmen
    ------
    2
    ------