JAEN

SQL

外部キー制約

CautionForeign keys

ALTER TABLE <t> ADD FOREIGN KEY (<col>) REFERENCES <p>(<id>);

Enforces referential integrity; the ON DELETE policy decides deletion behavior — choose it deliberately.

Examples

  • ALTER TABLE orders ADD CONSTRAINT fk_user FOREIGN KEY (user_id) REFERENCES users(id);Guarantee order→user integrity

Syntax

ALTER TABLE t ADD CONSTRAINT fk FOREIGN KEY (col) REFERENCES parent(id) [ON DELETE CASCADE];

Before you run it

Existing orphans block creation.

Related commands

The same reference, with no signal at all

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