Why do I need “OR NULL” in MySQL when counting rows with a condition

Question

There is a question about MySQL\'s COUNT() aggregate function that keeps popping into my head time to time. I would like to get some explanation to why it is working the way

Answer 1

It's because COUNT(expression) counts VALUES. In SQL theory, NULL is a STATE, not a VALUE and thus is it not counted. NULL is a state that means that field's value is unknown.

Now, when you write "value=4" this evaluates to boolean TRUE or FALSE. Since both TRUE and FALSE are VALUES, the result is 10.

When you add "OR NULL", you actually have "TRUE OR NULL" and "FALSE OR NULL". Now, "TRUE OR NULL" evaluates to TRUE, while "FALSE OR NULL" evaluates to NULL. Thus the result is 3, because you only have 3 values (and seven NULL states).