Is it possible to temporarily disable constraints in MySQL?
I have two Django models, each with a foreign key to the other one. Deleting instances of a model returns an error because of the foreign key constraint:
cursor.execute("DELETE FROM myapp_item WHERE n = %s", n)
transaction.commit_unless_managed() #a foreign key constraint fails here
cursor.execute("DELETE FROM myapp_style WHERE n = %s", n)
transaction.commit_unless_managed()
Is it possible to temporarily disable constraints and delete anyway?
ON DELETE SET NULL
? That would accomplish a similar thing and you wouldn't have to turn key checking on and off.
Try DISABLE KEYS
or
SET FOREIGN_KEY_CHECKS=0;
Make sure to
SET FOREIGN_KEY_CHECKS=1;
after.
To turn off foreign key constraint globally, do the following:
SET GLOBAL FOREIGN_KEY_CHECKS=0;
and remember to set it back when you are done
SET GLOBAL FOREIGN_KEY_CHECKS=1;
WARNING: You should only do this when you are doing single user mode maintenance. As it might resulted in data inconsistency. For example, it will be very helpful when you are uploading large amount of data using a mysqldump output.
SET FOREIGN_KEY_CHECKS
just changes the value for the current connection, while SET GLOBAL ..
changes the value for all connections, including future connections. If you just do SET FOREIGN..
in one window, then try to apply the statement in a different window (over a different connection), the value has not changed there. With GLOBAL
, the same variable has the same value for both connections.
ERROR 1228 (HY000): Variable 'foreign_key_checks' is a SESSION variable and can't be used with SET GLOBAL
I normally only disable foreign key constraints when I want to truncate a table, and since I keep coming back to this answer this is for future me:
SET FOREIGN_KEY_CHECKS=0;
TRUNCATE TABLE table;
SET FOREIGN_KEY_CHECKS=1;
Instead of disabling your constraint, permanently modify it to ON DELETE SET NULL. That will accomplish a similar thing and you wouldn't have to turn key checking on and off. Like so:
ALTER TABLE tablename1 DROP FOREIGN KEY fk_name1; //get rid of current constraints
ALTER TABLE tablename2 DROP FOREIGN KEY fk_name2;
ALTER TABLE tablename1
ADD FOREIGN KEY (table2_id)
REFERENCES table2(id)
ON DELETE SET NULL //add back constraint
ALTER TABLE tablename2
ADD FOREIGN KEY (table1_id)
REFERENCES table1(id)
ON DELETE SET NULL //add back other constraint
Have a read of this (http://dev.mysql.com/doc/refman/5.5/en/alter-table.html) and this (http://dev.mysql.com/doc/refman/5.5/en/create-table-foreign-keys.html).
FOREIGN_KEY_CHECKS
to 0 and put it back once the dirty work is done. Besides it might lock for writing your tables.
To turn off the foreign key constraint globally:
SET GLOBAL FOREIGN_KEY_CHECKS = 0;
And for the active foreign key constraint:
SET GLOBAL FOREIGN_KEY_CHECKS = 1;
A very simple solution with phpMyAdmin:
In your table, go to the SQL tab
After you edit the SQL command that you want to run, there is a check box next to GO, named 'Enable foreign key checks' .
Uncheck this check box and run your SQL. It will be automatically rechecked after executing.
SET FOREIGN_KEY_CHECKS=0; ..... SET FOREIGN_KEY_CHECKS=1;
didn't work for me in PHPMyAdmin because I forgot to uncheck the 'Enable foreign key checks' checkbox. In PHPMyAdmin you can skip these SET commands and just uncheck the checkbox.
For me just SET FOREIGN_KEY_CHECKS=0;
wasn't enough. I was still having a com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException
.
I had to add ALTER TABLE myTable DISABLE KEYS;
.
So:
SET FOREIGN_KEY_CHECKS=0;
ALTER TABLE myTable DISABLE KEYS;
DELETE FROM myTable;
ALTER TABLE myTable ENABLE KEYS;
SET FOREIGN_KEY_CHECKS=1;
If the key field is nullable, then you can also set the value to null before attempting to delete it:
cursor.execute("UPDATE myapp_item SET myapp_style_id = NULL WHERE n = %s", n)
transaction.commit_unless_managed()
cursor.execute("UPDATE myapp_style SET myapp_item_id = NULL WHERE n = %s", n)
transaction.commit_unless_managed()
cursor.execute("DELETE FROM myapp_item WHERE n = %s", n)
transaction.commit_unless_managed()
cursor.execute("DELETE FROM myapp_style WHERE n = %s", n)
transaction.commit_unless_managed()
In phpMyAdmin you can select multiple rows and can then click the delete action. You'll enter a screen which lists the delete queries. It looks like this:
https://i.stack.imgur.com/Y8Xgv.png
Please uncheck the "Enable foreign key checks" checkbox, and click on Yes to execute them.
This will enable you to delete rows even if there is an ON DELETE restriction constraint.
It's not a good idea to set a foreign key constraint to 0, because if you do, your database would not ensure it is not violating referential integrity. This could lead to inaccurate, misleading, or incomplete data.
You make a foreign key for a reason: because all the values in the child column shall be the same as a value in the parent column. If there are no foreign key constraints, a child row can have a value that is not in the parent row, which would lead to inaccurate data.
For instance, let's say you have a website for students to login and every student must register for an account as a user. You have one table for user ids, with user id as a primary key; and another table for student accounts, with student id as a column. Since every student must have a user id, it would make sense to make the student id from the student accounts table a foreign key that references the primary key user id in the user ids table. If there are no foreign key checks, a student could end up having a student id and no user id, which means a student can get an account without being a user, which is wrong.
Imagine if it happens to a large amount of data. That's why you need the foreign key check.
It's best to figure out what is causing the error. Most likely, you are trying to delete from a parent row without deleting from a child row. Try deleting from the child row before deleting from the parent row.
Success story sharing
disable keys
for InnodbDISABLE KEYS
. But you can accomplish what is requested in this question, using the other command shown:SET FOREIGN_KEY_CHECKS=0;
(I thought at first he was saying there was no way to accomplish this.)