What are comparison operators in MySQL, and how are they used with NULL values?

Any comparison with NULL returns NULL (unknown).

This includes expressions like: 5 = NULL, NULL <> 10, NULL > 0.

Even NULL = NULL returns NULL (because NULL means unknown, so two unknowns cannot be equal).

<=> is MySQL's NULL-safe comparison operator.

Returns 1 if both values are equal — including when both are NULL.

Returns 0 otherwise.

IN ignores NULL values in the list.

NOT IN returns NULL if the list contains a NULL — causing unexpected results.

NULL means unknown, so comparisons involving NULL result in NULL.

Use <=> for NULL-safe comparisons.

Beware of NULL inside NOT IN — it can make the whole condition NULL.

NULL does not behave like 0 or empty string.