MySQL DatabaseLesson 56Free preview

MySQL Foreign Key

This lesson is part of MySQL · All levels

Summary

FOREIGN KEY maintains the reference integrity by linking the columns in the child table to the primary key or unique key in the parent table. With this relationship, nonexistent referral data cannot be stored, such as orders for customers that have already been deleted. You can also set automatic actions by ON DELETE and ON UPDATE such as CASCADE or SET.

Key points

Foreign key refers to the primary key or unique key in other tables
The FK storage table is called the child table, the table referred to is called the parent table
ON DELETE CASCADE deletes the child line when the parent is deleted
ON DELETE SET NULL empties the child's FK column when the parent is deleted
ON DELETE RESTRICT/NO ACTION refuses to remove the parent as long as it is still referred

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