Why Can't I Drop a Table with Foreign Key Constraints Using Multiple Queries?

Bogus Foreign Key Constraint Fail

When attempting to drop a table named area from a database, an error message was encountered indicating a foreign key constraint failure. Despite having dropped all other tables that had foreign key relationships with area, the constraint continued to persist.

Possible Reasons

InnoDB, the storage engine used by the database, typically does not allow foreign keys to reference tables outside of the current schema. Therefore, it was puzzling why foreign key constraints were still preventing the deletion of the area table.

Solution

Upon further investigation, it was discovered that certain database tools, such as MySQL Query Browser or phpMyAdmin, open a new connection for each query. This behavior prevents the propagation of changes across multiple queries. To resolve the issue, it is necessary to execute all drop statements within a single query:

SET FOREIGN_KEY_CHECKS=0;  -- Disable foreign key checks
DROP TABLE my_first_table_to_drop;
DROP TABLE my_second_table_to_drop;
SET FOREIGN_KEY_CHECKS=1;  -- Re-enable foreign key checks

Disabling foreign key checks allows the deletion of tables that are involved in foreign key relationships without triggering the constraint error. The subsequent re-enabling of foreign key checks ensures the integrity of the database for future operations.

The above is the detailed content of Why Can't I Drop a Table with Foreign Key Constraints Using Multiple Queries?. For more information, please follow other related articles on the PHP Chinese website!