Error Cannot truncate a table referenced in a foreign key constraint mytest instance CONSTRAINT instance ibfk 1 FOREIGN KEY GroupID REFERENCES mytest mygroup ID

Question

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

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

My code:

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

Hello @kartik,

You cannot TRUNCATE a table that has FK constraints applied on it (TRUNCATE is not the same as DELETE).

To work around this, use either of these solutions:

Option 1:

1. Remove constraints
2. Perform TRUNCATE
3. Delete manually the rows that now have references to nowhere
4. Create constraints

Option 2: 

SET FOREIGN_KEY_CHECKS = 0; 
TRUNCATE table $table_name; 
SET FOREIGN_KEY_CHECKS = 1;

Hope it helps!!
Thank you!!