Drop foreign key only if it exists

Question

I'm on a MySQL database.

I'm doing this, but it doesn't work.

ALTER TABLE `object` DROP FOREIGN KEY IF EXISTS `object_ibfk_1`;

I've tried to put this IF EXISTS wherever I could. How can check if foreign key is exists before drop it?

Answer 1

Here is a workaround for the DROP FOREIGN KEY IF EXISTS, that is missing in MySQL and MariaDB versions before v10.1.4. You can also use it for every other statement you want, that should be depend on the existence of an FOREIGN KEY (e.g. for SELECT "info: foreign key exists." like in the example below).

-- DROP FOREIGN KEY IF EXISTS
SELECT
    COUNT(*)
INTO
    @FOREIGN_KEY_my_foreign_key_ON_TABLE_my_table_EXISTS
FROM
    `information_schema`.`table_constraints`
WHERE
    `table_schema` = 'my_database'
    AND `table_name` = 'my_table'
    AND `constraint_name` = 'my_foreign_key'
    AND `constraint_type` = 'FOREIGN KEY'
;
-- SELECT @FOREIGN_KEY_my_foreign_key_ON_TABLE_my_table_EXISTS;
SET @statement := IF(
    @FOREIGN_KEY_my_foreign_key_ON_TABLE_my_table_EXISTS > 0,
    -- 'SELECT "info: foreign key exists."',
    'ALTER TABLE my_table DROP FOREIGN KEY my_foreign_key',
    'SELECT "info: foreign key does not exist."'
);
PREPARE statement FROM @statement;
EXECUTE statement;