In a Zen-like manner, "NULL" is never equal to anything, even itself. Therefore comparisons using equality operators will always return
False, even when the value actually IS NULL.
For that reason, comparison operators should never be used to make comparisons with NULL; IS NULL and IS NOT
NULL should be used instead. This extends as well to empty string (""), which is equivalent to NULL for some database engines.
Noncompliant code example
UPDATE books
SET title = 'unknown'
WHERE title = NULL -- Noncompliant
Compliant solution
UPDATE books
SET title = 'unknown'
WHERE title IS NULL
