foreign keys – How to remove constraints from my MySQL table?

The Question :

263 people think this question is useful

I want to remove constraints from my table. My query is:

ALTER TABLE `tbl_magazine_issue` 
DROP CONSTRAINT `FK_tbl_magazine_issue_mst_users`

But I got an error:

#1064 – You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘constraint FK_tbl_magazine_issue_mst_users‘ at line 1

The Question Comments :
  • It is worth noting that if you created a CHECK constraint, there is no need to drop it because no actual constraint is created. You can select from information_schema.table_constraints to verify, and you can even run the add constraint over and over again without any error. MySQL does not support CHECK constraints but allows the SQL intended to create them (without actually creating the constraints).
  • feature request: Foreign keys: DROP CONSTRAINT as a compatibility alias for DROP FOREIGN KEY
  • Possible duplicate of Remove Primary Key in MySQL
  • Your syntax is perfectly valid and now it is supported demo

The Answer 1

437 people think this answer is useful

Mysql has a special syntax for dropping foreign key constraints:

ALTER TABLE tbl_magazine_issue
  DROP FOREIGN KEY FK_tbl_magazine_issue_mst_users

The Answer 2

57 people think this answer is useful

I had the same problem and I got to solve with this code:

ALTER TABLE `table_name` DROP FOREIGN KEY `id_name_fk`;
ALTER TABLE `table_name` DROP INDEX  `id_name_fk`;

The Answer 3

26 people think this answer is useful

There is no such thing as DROP CONSTRAINT in MySQL. In your case you could use DROP FOREIGN KEY instead.

The Answer 4

14 people think this answer is useful

If the constraint is not a foreign key, eg. one added using ‘UNIQUE CONSTRAINT (colA, colB)’ then it is an index that can be dropped using ALTER TABLE ... DROP INDEX ...

The Answer 5

9 people think this answer is useful

To add a little to Robert Knight’s answer, since the title of the post itself doesn’t mention foreign keys (and since his doesn’t have complete code samples and since SO’s comment code blocks don’t show as well as the answers’ code blocks), I’ll add this for unique constraints. Either of these work to drop the constraint:

ALTER TABLE `table_name` DROP KEY `uc_name`;

or

ALTER TABLE `table_name` DROP INDEX `uc_name`;

The Answer 6

9 people think this answer is useful

Also nice, you can temporarily disable all foreign key checks from a mysql database: SET FOREIGN_KEY_CHECKS=0; And to enable it again: SET FOREIGN_KEY_CHECKS=1;

The Answer 7

4 people think this answer is useful

Some ORM’s or frameworks use a different naming convention for foreign keys than the default FK_[parent table]_[referenced table]_[referencing field], because they can be altered.

Laravel for example uses [parent table]_[referencing field]_foreign as naming convention. You can show the names of the foreign keys by using this query, as shown here:

SELECT CONSTRAINT_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE 
WHERE REFERENCED_TABLE_SCHEMA = '<database>' AND REFERENCED_TABLE_NAME = '<table>';

Then remove the foreign key by running the before mentioned DROP FOREIGN KEY query and its proper name.

The Answer 8

2 people think this answer is useful

For those that come here using MariaDB:

Note that MariaDB allows DROP CONSTRAINT statements in general, for example for dropping check constraints:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

https://mariadb.com/kb/en/library/alter-table/

The Answer 9

1 people think this answer is useful
  1. Go to structure view of the table
  2. You will see 2 option at top a.Table structure b.Relation view.
  3. Now click on Relation view , here you can drop your foreign key constraint. You will get all relation here.

The Answer 10

1 people think this answer is useful

The simplest way to remove constraint is to use syntax ALTER TABLE tbl_name DROP CONSTRAINT symbol; introduced in MySQL 8.0.19:

As of MySQL 8.0.19, ALTER TABLE permits more general (and SQL standard) syntax for dropping and altering existing constraints of any type, where the constraint type is determined from the constraint name

ALTER TABLE tbl_magazine_issue DROP CONSTRAINT FK_tbl_magazine_issue_mst_users;

db<>fiddle demo

The Answer 11

0 people think this answer is useful

There is no DROP CONSTRAINT In MySql. This work like magic in mysql 5.7

ALTER TABLE answer DROP KEY const_name;

The Answer 12

-4 people think this answer is useful

this will works on MySQL to drop constraints

alter table tablename drop primary key;

alter table tablename drop foreign key;

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