What does a CHECK constraint do?

Question

Accepted Answer

A CHECK constraint lets you write an arbitrary condition that every inserted or updated value must pass - for example CHECK (age >= 0) or CHECK (status IN ('active', 'inactive')). If the condition evaluates to FALSE, the write is rejected. NULL is the subtle edge case: if the expression evaluates to NULL (unknown), most databases let the row through rather than rejecting it. Use NOT NULL alongside CHECK when you want to block NULLs too.