KITFORMA · EDITORIAL ANSWER
Why does a CHECK constraint still allow a NULL value?
KitForma editorial guide. A CHECK constraint accepts a result that is true or unknown; it rejects false. A comparison involving NULL can therefore pass without proving the intended value exists.
Step-by-step answer
A CHECK constraint accepts a result that is true or unknown; it rejects false. A comparison involving NULL can therefore pass without proving the intended value exists.
Combine a value rule such as
CHECK (quantity > 0) with NOT NULL when presence is required. Test positive, zero, negative and NULL values directly against the database. Do not rely only on a form validator, because imports and background jobs may bypass it. Cross-row or cross-table invariants generally need a different design; a simple CHECK should not pretend to validate changing external state.Sources and verification
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