Specifies a predicate that SQL evaluates to generate an
    intermediate result table. SQL evaluates the predicate for each
    group of the intermediate result table created by a preceding
    clause. The groups of that table for which the predicate is true
    become another intermediate result table to which SQL applies the
    select list for evaluation.

    If the clause preceding the HAVING clause is a GROUP BY
    clause, then the predicate is evaluated for each group in the
    intermediate result table. The HAVING clause affects groups just
    as the WHERE clause affects individual rows.

    If the HAVING clause is not preceded by a GROUP BY clause, SQL
    evaluates the predicate for all the rows in the intermediate
    result table as a single group.

    SQL restricts which expressions you can specify in the predicate
    of a HAVING clause. A column name or expression in a HAVING
    predicate must meet one of the following criteria:

    o  It must also appear in the GROUP BY clause.

    o  It must be specified within an aggregate function.

    o  It must be an outer reference (possible only if the HAVING
       clause is part of a column select expression).

    For instance, the following statement is invalid. It has a
    HAVING clause without a GROUP BY clause, which means that any
    column names in the HAVING clause must be part of a function
    (because there is no outer query, the column names cannot be
    outer references).

    SQL> SELECT LAST_NAME, FIRST_NAME FROM EMPLOYEES
    cont> HAVING FIRST_NAME = 'Bob';
    %SQL-F-NOTGROFLD, Column FIRST_NAME cannot be referred to in
    the select list or HAVING clause because it is not in the GROUP BY clause