What is MySQL\'s Default ON DELETE Behavior?
Posted on 2025-03-24
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MySQL's Default ON DELETE Behavior
When working with databases, understanding the behavior of foreign key constraints is crucial for maintaining data integrity. MySQL provides various options for specifying the action to be taken when a row in a parent table is deleted, one of which is the default behavior.
Default Behavior
MySQL's default ON DELETE behavior is to enforce referential integrity, preventing any change to the parent table that would break the relationship with child tables. This is achieved through the NO ACTION or RESTRICT options. Both these options essentially prohibit the deletion if dependent rows exist in child tables.
Other Options
MySQL offers additional options to handle ON DELETE events:
- SET NULL: Deletes the parent row and sets the foreign key in the child table to NULL, if not declared NOT NULL.
- CASCADE: Deletes the parent row and automatically deletes dependent rows in the child tables.
- SET DEFAULT: This option is rejected by InnoDB and cannot be used.
Therefore, to answer the question:
- Yes, the premise is correct. MySQL's default ON DELETE behavior is NO ACTION (or RESTRICT), preventing database changes that violate foreign key constraints.
- NO ACTION and RESTRICT are synonymous, and they apply when no ON DELETE clause is specified.
- SET NULL allows parent row deletion and sets foreign keys to NULL.
- CASCADE deletes parent and dependent rows.
- SET DEFAULT should not be used as it is rejected by InnoDB.
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