null – Federico Razzoli

What does NULL mean in SQL?

Does NULL mean “no value”? Or does it mean “an unknown value”? Its semantics and history are ambiguous and inconsistent.

It is quite common to use an equality condition and an IS NULL condition on the same columns. Unfortunately, this could make queries very slow.

Any comparison involving at least one NULL operand will return NULL. However, SQL supports operators to easily write NULL-safe comparisons.