I use Constraints in my MySQL Database. But now it gives me a headache when I try to delete an entry on which other entries have a foreign-key relationship. I always get this error:
Cannot delete or update a parent row: a foreign key constraint fails
Can I pass the delete-statement any parameter or anything, so that it recursivly deletes all rows that have a foreign-key relationship to the row I'm trying to delete?
UPDATE: Have now made this into a blog post: https://stevettt.blogspot.co.uk/2018/02/how-to-automate-deletion-of-rows-in.html
I've written a stored procedure that will recursively delete from all foreign key - linked tables (without needing to turn off foreign key checks or turn on cascade deletes). The implementation has some complexity but can be treated as a "black box": Simply specify the name of the schema (database), table and a WHERE clause to restrict the records to be deleted and it will do the rest.
Demo
Rextester online demo: http://rextester.com/MDMRA15991
SQL
-- ------------------------------------------------------------------------------------
-- USAGE
-- ------------------------------------------------------------------------------------
-- CALL delete_recursive(<schema name>, <table name>, <WHERE clause>, <delete flag>);
-- where:
-- <schema name> is the name of the MySQL schema
-- <table name> is the name of the base table to delete records from
-- <WHERE clase> is a SQL WHERE clause to filter which records that are to be deleted
-- <delete flag> is either TRUE or FALSE: If TRUE, the records *will* be deleted.
-- If FALSE, the SQL will be output without actually deleting anything.
-- Example:
-- CALL delete_recursive('mydb', 'mytable', 'WHERE mypk IN (1, 2, 3)', TRUE);
DROP PROCEDURE IF EXISTS delete_recursive;
DELIMITER //
CREATE PROCEDURE delete_recursive(schema_name VARCHAR(64),
tbl_name VARCHAR(64),
where_clause TEXT,
do_delete BIT)
BEGIN
DECLARE next_schema_name, next_tbl_name VARCHAR(64);
DECLARE from_clause, next_where_clause, next_col_names, ref_col_names TEXT;
DECLARE done INT DEFAULT FALSE;
DECLARE cursor1 CURSOR FOR
SELECT TABLE_SCHEMA, TABLE_NAME, COLUMN_NAMES, REF_COLUMN_NAMES FROM temp_kcu;
DECLARE cursor2 CURSOR FOR
SELECT table_schema, table_name, where_sql FROM temp_deletes ORDER BY id;
DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
-- Set maximum recursion depth
SET @@SESSION.max_sp_recursion_depth = 255;
-- Increment current recursion depth since the stored procedure has been entered.
SET @recursion_depth = IFNULL(@recursion_depth + 1, 0);
-- Create temporary table for storing the deletes if it doesn't already exist
IF @recursion_depth = 0 THEN
DROP TEMPORARY TABLE IF EXISTS temp_deletes;
CREATE TEMPORARY TABLE temp_deletes (
id INT NOT NULL AUTO_INCREMENT,
table_schema VARCHAR(64),
table_name VARCHAR(64),
where_sql TEXT,
Notes TEXT,
PRIMARY KEY(id)
);
END IF;
-- Construct FROM clause (including the WHERE clause) for this table.
SET from_clause =
CONCAT(' FROM ', schema_name, '.', tbl_name, ' WHERE ', where_clause);
-- Find out whether there are any foreign keys to this table
SET @query = CONCAT('SELECT COUNT(*) INTO @count', from_clause);
PREPARE stmt FROM @query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
IF @count > 0 THEN
-- There are foriegn keys to this table so all linked rows must be deleted first:
-- Firstly, fill a temporary table with the foreign key metadata.
DROP TEMPORARY TABLE IF EXISTS temp_kcu;
SET @query = CONCAT(
'CREATE TEMPORARY TABLE temp_kcu AS ',
'SELECT TABLE_SCHEMA, TABLE_NAME, ',
'GROUP_CONCAT(CONCAT(COLUMN_NAME) SEPARATOR '', '') AS COLUMN_NAMES, ',
'GROUP_CONCAT(CONCAT(REFERENCED_COLUMN_NAME) SEPARATOR '', '')
AS REF_COLUMN_NAMES ',
'FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE ',
'WHERE REFERENCED_TABLE_SCHEMA = ''', schema_name,
''' AND REFERENCED_TABLE_NAME = ''', tbl_name, ''' ',
'GROUP BY CONSTRAINT_NAME');
PREPARE stmt FROM @query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
-- Loop through all foreign keys to this table using a cursor.
OPEN cursor1;
read_loop: LOOP
FETCH cursor1 INTO next_schema_name, next_tbl_name, next_col_names,
ref_col_names;
IF done THEN
-- No more rows so exit the loop.
LEAVE read_loop;
END IF;
-- Recursively call the stored procedure to delete linked rows
-- for this foreign key.
IF INSTR(next_col_names, ',') = 0 THEN
SET next_where_clause = CONCAT(
next_col_names, ' IN (SELECT ', ref_col_names, from_clause, ')');
ELSE
SET next_where_clause = CONCAT(
'(', next_col_names, ') IN (SELECT ', ref_col_names, from_clause, ')');
END IF;
CALL delete_recursive(
next_schema_name, next_tbl_name, next_where_clause, do_delete);
END LOOP;
CLOSE cursor1;
END IF;
-- Build the DELETE statement
SET @query = CONCAT(
'DELETE FROM ', schema_name, '.', tbl_name, ' WHERE ', where_clause);
-- Get the number of primary key columns
SET @pk_column_count = (SELECT COUNT(*)
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_SCHEMA = schema_name
AND TABLE_NAME = tbl_name
AND CONSTRAINT_NAME = 'PRIMARY');
IF @pk_column_count = 0 THEN
-- No primary key so just output the number of rows to be deleted
SET @query = CONCAT(
'SET @notes = CONCAT(''No primary key; number of rows to delete = '',
(SELECT COUNT(*) FROM ', schema_name, '.', tbl_name, ' WHERE ',
where_clause, '))');
PREPARE stmt FROM @query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
ELSEIF @pk_column_count = 1 THEN
-- 1 primary key column.
-- Output the primary keys of the records to be deleted
SET @pk_column = (SELECT COLUMN_NAME
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_SCHEMA = schema_name
AND TABLE_NAME = tbl_name
AND CONSTRAINT_NAME = 'PRIMARY');
SET @pk_column_csv = CONCAT('CONCAT('''''''', ', @pk_column, ', '''''''')');
SET @query = CONCAT(
'SET @notes = (SELECT CONCAT(''', @pk_column, ' IN ('', GROUP_CONCAT(',
@pk_column_csv, ' SEPARATOR '', ''), '')'') FROM ',
schema_name, '.', tbl_name, ' WHERE ', where_clause, ')');
PREPARE stmt FROM @query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
ELSE
-- Multiple primary key columns.
-- Output the primary keys of the records to be deleted.
SET @pk_columns = (SELECT GROUP_CONCAT(COLUMN_NAME SEPARATOR ', ')
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_SCHEMA = schema_name
AND TABLE_NAME = tbl_name
AND CONSTRAINT_NAME = 'PRIMARY');
SET @pk_columns_csv = (SELECT CONCAT('CONCAT(''('''''', ', GROUP_CONCAT(COLUMN_NAME
SEPARATOR ', '''''', '''''', '), ', '''''')'')')
FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE TABLE_SCHEMA = schema_name
AND TABLE_NAME = tbl_name
AND CONSTRAINT_NAME = 'PRIMARY');
SET @query = CONCAT(
'SET @notes = (SELECT CONCAT(''(', @pk_columns,
') IN ('', GROUP_CONCAT(', @pk_columns_csv, ' SEPARATOR '', ''), '')'') FROM ',
schema_name, '.', tbl_name, ' WHERE ', where_clause, ')');
PREPARE stmt FROM @query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END IF;
IF @notes IS NULL THEN
SET @notes = 'No affected rows.';
END IF;
-- Save details of the DELETE statement to be executed
INSERT INTO temp_deletes (table_schema, table_name, where_sql, Notes)
VALUES (schema_name, tbl_name, where_clause, @notes);
IF @recursion_depth = 0 THEN
-- Output the deletes.
SELECT CONCAT('DELETE FROM ', schema_name, '.', table_name,
' WHERE ', where_sql) `SQL`,
Notes
FROM temp_deletes ORDER BY id;
IF do_delete THEN
-- Perform the deletes: Loop through all delete queries using a cursor.
SET done = FALSE;
OPEN cursor2;
read_loop: LOOP
FETCH cursor2 INTO schema_name, tbl_name, where_clause;
IF done THEN
-- No more rows so exit the loop.
LEAVE read_loop;
END IF;
SET @query = CONCAT(
'DELETE FROM ', schema_name, '.', tbl_name, ' WHERE ', where_clause);
PREPARE stmt FROM @query;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END LOOP;
CLOSE cursor2;
END IF;
-- Tidy up
DROP TEMPORARY TABLE IF EXISTS temp_deletes;
END IF;
-- Decrement current recursion depth since the stored procedure is being exited.
SET @recursion_depth = @recursion_depth - 1;
END;//
DELIMITER ;
Limitations
Look at this:
In what order are ON DELETE CASCADE constraints processed?
But I think you can use ON DELETE CASCADE from my research. If I am wrong I am sure the community will let me know. I believe you will have to alter your tables, if possible.
Also see this:
Cannot delete or update a parent row: a foreign key constraint fails
