How to truncate a foreign key constrained table?

Question

Why doesn't a TRUNCATE on mygroup work? Even though I have ON DELETE CASCADE SET I get:

ERROR 1701 (42000): Cannot truncate a table referenced in a foreign key constraint (mytest.instance, CONSTRAINT instance_ibfk_1 FOREIGN KEY (GroupID) REFERENCES mytest.mygroup (ID))

drop database mytest; create database mytest; use mytest;  CREATE TABLE mygroup (    ID    INT NOT NULL AUTO_INCREMENT PRIMARY KEY ) ENGINE=InnoDB;  CREATE TABLE instance (    ID           INT NOT NULL AUTO_INCREMENT PRIMARY KEY,    GroupID      INT NOT NULL,    DateTime     DATETIME DEFAULT NULL,     FOREIGN KEY  (GroupID) REFERENCES mygroup(ID) ON DELETE CASCADE,    UNIQUE(GroupID) ) ENGINE=InnoDB; 

Answer 1

Yes you can:

SET FOREIGN_KEY_CHECKS = 0;  TRUNCATE table1; TRUNCATE table2;  SET FOREIGN_KEY_CHECKS = 1; 

With these statements, you risk letting in rows into your tables that do not adhere to the FOREIGN KEY constraints.