NULL is a constant and it is False. For the purposes of GROUP BY and DISTINCT, the grouping comparison is 'IS' not '==' so NULL compares equal to NULL and all NULL form a single group just like any other value.
NULL is a constant and it is False. For the purposes of GROUP BY and DISTINCT, the grouping comparison is 'IS' not '==' so NULL compares equal to NULL and all NULL form a single group just like any other value.