问题
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 'constraintFK_tbl_magazine_issue_mst_users' at line 1
回答1:
Mysql has a special syntax for dropping foreign key constraints:
ALTER TABLE tbl_magazine_issue
DROP FOREIGN KEY FK_tbl_magazine_issue_mst_users
回答2:
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`;
回答3:
There is no such thing as DROP CONSTRAINT in MySQL. In your case you could use DROP FOREIGN KEY instead.
回答4:
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 ...
回答5:
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;
回答6:
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`;
回答7:
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.
回答8:
- Go to structure view of the table
- You will see 2 option at top a.Table structure b.Relation view.
- Now click on Relation view , here you can drop your foreign key constraint. You will get all relation here.
回答9:
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/
回答10:
this will works on MySQL to drop constraints
alter table tablename drop primary key;
alter table tablename drop foreign key;
来源：https://stackoverflow.com/questions/14122031/how-to-remove-constraints-from-my-mysql-table