What are the Different ON DELETE Behaviors in MySQL and How do they Work?

MySQL's Default ON DELETE Behavior: A Comprehensive Guide

In MySQL, the ON DELETE behavior defines the action to be taken on a child row when its related parent row is deleted. Understanding this behavior is crucial for maintaining database integrity.

The Five Options:

MySQL provides five possible ON DELETE behaviors:

• SET NULL: Sets the foreign key column to NULL in the child row.
• NO ACTION: Prevents any database change that would violate a foreign key constraint.
• RESTRICT: Equivalent to NO ACTION; prevents foreign key violations.
• CASCADE: Automatically deletes or updates the child row when the parent row is deleted or updated.
• SET DEFAULT: Sets the foreign key column to its default value, but is not supported by InnoDB.

Default Behavior:

The default ON DELETE behavior is NO ACTION, which means that MySQL will prevent any deletion or update that would break a foreign key constraint. If no ON DELETE clause is specified, the default behavior is assumed.

Other Options:

• SET NULL: This option allows the parent row to be deleted, and the foreign key column in the child row is set to NULL.
• CASCADE: Performs a cascading operation, deleting or updating the child row along with the parent row.
• SET DEFAULT: This option is not supported by InnoDB, meaning it is not a valid behavior.

Understanding NO ACTION and RESTRICT:

NO ACTION and RESTRICT are essentially synonymous. Both options prevent foreign key violations by rejecting any operation that would break a constraint.

Conclusion:

The ON DELETE behavior is a critical aspect of database design, as it determines how child rows are affected when their parent rows are modified or deleted. By understanding the five available options and the default behavior, developers can ensure the integrity and consistency of their MySQL databases.

The above is the detailed content of What are the Different ON DELETE Behaviors in MySQL and How do they Work?. For more information, please follow other related articles on the PHP Chinese website!