Indicates the value expressions that SQL uses for organizing the
    intermediate result table from the WHERE clause, if specified, or
    the FROM clause. These groups of rows containing the same value
    are also called control breaks.

    For the first expression specified in the GROUP BY clause, SQL
    orders the rows of the preceding intermediate result table into
    groups whose rows all have the same value for the specified
    expression. If a second expression is specified in the GROUP
    BY clause, SQL then groups rows within each main group by values
    of the second expression. SQL groups any additional columns in
    the GROUP BY clause in a similar manner.

    All null values for a column name in the GROUP BY clause are
    grouped together.

    Each group is treated as the source for the values of a single
    row of the result table.

    Because all rows of a group have the same value for the value
    expression specified in the GROUP BY clause, references within
    value expressions or predicates to that column specify a single
    value.