Mysql: How to truncate a foreign key constrained table?

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;

 Answers

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. Both present risks of damaging the data integrity.

Option 1:

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

Option 2: suggested by user447951 in their answer

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

---

I would simply do it with :

DELETE FROM mytest.instance;
ALTER TABLE mytest.instance AUTO_INCREMENT = 1;

---

you can do

DELETE FROM `mytable` WHERE `id` > 0

---

Option 1:

Option 1: which does not risk damage to data integrity:

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

The tricky part is Removing constraints, so I want to tell you how, in case someone needs to know how to do that:

1. Run SHOW CREATE TABLE <Table Name> query to see what is your FOREIGN KEY's name (Red frame in below image):
2. Run ALTER TABLE <Table Name> DROP FOREIGN KEY <Foreign Key Name>. This will remove the foreign key constraint.
3. Drop the associated Index (through table structure page), and you are done.

to re-create foreign keys:

ALTER TABLE <Table Name>
ADD FOREIGN KEY (<Field Name>) REFERENCES <Foreign Table Name>(<Field Name>);