what does “delete from table where NULL = NULL” means?

Question

What does delete from table where NULL = NULL mean?

Answer 1

It means don't delete anything, because NULL is never equal to anything. Or maybe it means "don't delete anything unless the user's DBMS really sucks, in which case delete it all out of spite".

Seriously though, that kind of construct usually comes about when a WHERE clause is procedurally generated -- rather than creating a special case for "do nothing", sometimes it's simpler just to generate a WHERE clause that causes the database to do nothing. I've usually seen "WHERE 0 = 1" though, which is less ambiguous.