Interview › NULL three-valued logic

SQL interview: why WHERE col <> 'x' dropped the NULLs

The filter is not false. It is unknown. Unknown rows vanish.

Prompt. All orders that are not Cancelled, including those with a NULL status.

Hint: WHERE status <> 'Cancelled' OR status IS NULL

Your T-SQL

SELECT * FROM Orders
WHERE status <> 'Cancelled' OR status IS NULL;

-- or
WHERE ISNULL(status, '') <> 'Cancelled';

ISNULL in a WHERE can kill a seek. The OR IS NULL form is the honest one.

Then write it on a ticket

Monday is free. A colleague asks, you type, we mark it.