Delete all foreign keys in mysql database - mysql

Question

I can see that there are some questions [1,2] that already ask for this, but where the solution didn't contain a complete SQL script to do this task.
I have a situation where it would be very helpful to delete all foreign keys using SQL only.
Currently I'm trying to solve this with a stored procedure and a cursor as follows:
-- No automatic commits:
DROP PROCEDURE IF EXISTS removeConstraints;
-- Magic to happen soon:
DELIMITER |
CREATE PROCEDURE removeConstraints()
BEGIN
SET AUTOCOMMIT=0;
SET FOREIGN_KEY_CHECKS=0;
-- https://dev.mysql.com/doc/refman/5.0/en/cursors.html
-- https://stackoverflow.com/questions/1745165/looping-over-result-sets-in-mysql
-- https://mariadb.com/kb/en/mariadb/cursor-overview/
DECLARE done INT DEFAULT FALSE;
DECLARE s VARCHAR(1024) DEFAULT '';
DECLARE cur CURSOR FOR SELECT CONCAT('ALTER TABLE ',TABLE_NAME, ' DROP FOREIGN KEY ', CONSTRAINT_NAME, ';')
FROM INFORMATION_SCHEMA.Key_COLUMN_USAGE
WHERE REFERENCED_TABLE_NAME IS NOT NULL;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
OPEN cur;
read_loop: LOOP
FETCH cur INTO s;
IF done THEN
LEAVE read_loop;
END IF;
PREPARE stmt FROM #s;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP;
CLOSE cur;
SET FOREIGN_KEY_CHECKS=1;
COMMIT;
SET AUTOCOMMIT=1;
END |
DELIMITER ;
-- Do magic:
CALL removeConstraints();
-- Cleanup:
DROP PROCEDURE removeConstraints;
Sadly this produces the following error message:
ERROR 1064 (42000) at line 5: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'DECLARE done INT DEFAULT FALSE;
DECLARE s VARCHAR(1024) DEFAULT '';
DECLARE ' at line 8
With the input from Ravinder Reddy I've now updated the DECLARE parts right after the BEGIN to look like this:
CREATE PROCEDURE removeConstraints()
BEGIN
DECLARE done INT DEFAULT FALSE;
DECLARE s VARCHAR(1024) DEFAULT '';
DECLARE cur CURSOR FOR
SELECT DISTINCT CONCAT('ALTER TABLE ',TABLE_NAME,' DROP FOREIGN KEY ',CONSTRAINT_NAME,';')
FROM INFORMATION_SCHEMA.Key_COLUMN_USAGE
WHERE TABLE_SCHEMA=DATABASE()
AND REFERENCED_TABLE_NAME IS NOT NULL;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
SET AUTOCOMMIT=0;
SET FOREIGN_KEY_CHECKS=0;
but when I try to execute the procedure I still get an error:
MariaDB [v4]> CALL removeConstraints();
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'NULL' at line 1
I've also tried to use a different SELECT statement like:
SELECT CONCAT('ALTER TABLE ',TABLE_NAME,' DROP FOREIGN KEY ',CONSTRAINT_NAME,';')
FROM INFORMATION_SCHEMA.Key_COLUMN_USAGE
WHERE TABLE_SCHEMA=DATABASE()
AND CONSTRAINT_NAME != 'PRIMARY'
AND CONSTRAINT_NAME IS NOT NULL
AND TABLE_NAME IS NOT NULL;
…but it didn't help.
I've got it working now by changing the code so that the CONCAT happens later.
DROP PROCEDURE IF EXISTS removeConstraints;
-- Magic to happen soon:
DELIMITER |
CREATE PROCEDURE removeConstraints()
BEGIN
DECLARE done INT DEFAULT FALSE;
DECLARE tName VARCHAR(64);
DECLARE cName VARCHAR(64);
DECLARE cur CURSOR FOR
SELECT DISTINCT TABLE_NAME, CONSTRAINT_NAME
FROM INFORMATION_SCHEMA.Key_COLUMN_USAGE
WHERE TABLE_SCHEMA=DATABASE()
AND REFERENCED_TABLE_NAME IS NOT NULL;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
SET AUTOCOMMIT=0;
SET FOREIGN_KEY_CHECKS=0;
OPEN cur;
read_loop: LOOP
FETCH cur INTO tName, cName;
IF done THEN
LEAVE read_loop;
END IF;
SET #sql = CONCAT('ALTER TABLE ',tName,' DROP FOREIGN KEY ',cName,';');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP;
CLOSE cur;
SET FOREIGN_KEY_CHECKS=1;
COMMIT;
SET AUTOCOMMIT=1;
END |
DELIMITER ;
-- Do magic:
CALL removeConstraints();
-- Cleanup:
DROP PROCEDURE removeConstraints;

Answer 1

All DECLARE statements must be on top in a BEGIN - END block.
And all other statements should follow them.
In your code, you have SET statements defined before DECLARE statements.
Move those statements to below of DECLARE statements.
Sample:
-- https://dev.mysql.com/doc/refman/5.0/en/cursors.html
-- https://stackoverflow.com/questions/1745165/looping-over-result-sets-in-mysql
-- https://mariadb.com/kb/en/mariadb/cursor-overview/
DECLARE done INT DEFAULT FALSE;
DECLARE s VARCHAR(1024) DEFAULT '';
DECLARE cur CURSOR FOR
SELECT CONCAT('ALTER TABLE ',TABLE_NAME,' DROP FOREIGN KEY ',CONSTRAINT_NAME,';')
FROM INFORMATION_SCHEMA.Key_COLUMN_USAGE
WHERE REFERENCED_TABLE_NAME IS NOT NULL;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
SET AUTOCOMMIT=0;
SET FOREIGN_KEY_CHECKS=0;
Document Reference:
DECLARE Syntax
DECLARE is permitted only inside a BEGIN ... END compound statement
and must be at its start, before any other statements.
Declarations must follow a certain order. Cursor declarations must
appear before handler declarations. Variable and condition
declarations must appear before cursor or handler declarations