The GROUP BY and HAVING Clauses

Purpose:  To apply aggregate operators to each of several groups of records.

Example:  Find the age of the youngest sailor for each rating level.
                 Should you repeat the following statements?
                         SELECT MIN (S.age)
                         FROM     Sailors S
                         WHERE  S.rating = i;     The i could be 1, 2, ..., 10.

Solution:
                              SELECT       S2.rating, MIN(S2.age)
                              FROM          Sailors S2
                              GROUP BY  S2.rating;
Example:  Find the age of the youngest sailor who is eligible to vote
                (i.e. is at least 18 years old) for each rating with at least two such sailors.

                         SELECT        S.rating,  MIN (S.age) AS minage
                    FROM           Sailors S
                    WHERE        S.age >= 18
                    GROUP BY  S.rating
                    HAVING     COUNT (*) > 1 ;