Rules the database enforces.
WHAT TYPES EXIST
Not null Unique Primary key Foreign key Check constraints Default values
WHAT NOT NULL ENFORCES
That a value must be present.
WHY IT MATTERS
Absent values propagate through calculations and comparisons unexpectedly.
WHAT UNIQUE ENFORCES
That no two rows share a value.
WHAT TO BE CAREFUL WITH
Unique constraints and absent values, which are frequently permitted more than once.
WHAT A CHECK CONSTRAINT ENFORCES
An arbitrary condition on the row.
WHERE IT HELPS
Amounts that cannot be negative Dates that must follow other dates Status values from a permitted set
WHAT VERSIONS SUPPORT THEM
Recent MySQL, and PostgreSQL long before.
WHY THAT MATTERS
Older MySQL accepted the syntax and ignored it entirely.
WHAT TO VERIFY
That your version actually enforces them.
WHAT FOREIGN KEYS ENFORCE
That referenced rows exist.
WHAT THEIR ACTIONS CONTROL
What happens on delete or update of the parent.
WHAT CASCADING DELETE DOES
Removes children automatically.
WHY IT DESERVES CAUTION
A single delete can remove far more than intended.
WHAT TO PREFER USUALLY
Restricting deletion, and handling it explicitly.
WHAT TO NEVER DO
Disable constraints permanently to make an import work.
