Code example sharing for MySQL to ignore foreign key constraints when deleting a table

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Deleting tables is not particularly common, especially for tables with foreign key associations, so you must be more careful when deleting. However, during the development process, it is common to find problems with the Schema design and to delete all tables in the existing database and recreate them. In addition, during testing, it is also necessary to recreate all tables in the database. Of course, many automated tools can also do such things.

When deleting a table, you sometimes encounter this error message:

ERROR 1217 (23000): Cannot delete or update a parent row: a foreign key constraint fails

This is because the fields in the table you are trying to delete are It is used as a foreign key to other tables, so the table with the foreign key (child table) must be deleted before deleting this table (parent table). In other words, the process of deleting a table needs to be the same as the process of creating a table.

But this is often unacceptable. On the one hand, if there are too many tables, manual sorting is a bit unacceptable; on the other hand, there is no automatic tool for sorting (in fact, it is not impossible). Therefore, MySQL provides a variableFOREIGN_KEY_CHECKS to set whether to check foreign key constraints when necessary.

It is generally recommended to do this:

First, automatically generate all DROP statements and replace MyDatabaseName with your database name:

SELECT concat('DROP TABLE IF EXISTS ', table_name, ';')
FROM information_schema.tables
WHERE table_schema = 'MyDatabaseName';

Then, add the following statements to set the FOREIGN_KEY_CHECKS variable before and after the generated code:

SET FOREIGN_KEY_CHECKS = 0
-- DROP语句
SET FOREIGN_KEY_CHECKS = 1;

However, it doesn’t matter if you forget the last sentence, this variable is based on Session , that is to say, when you close the client and re-establish the connection, this variable will return to the default value. If you need not to check foreign key constraints in the global scope (this situation will be relatively rare), you can do this:

SET GLOBAL FOREIGN_KEY_CHECKS = 0;

or

set @@global.FOREIGN_KEY_CHECKS = 0;

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