How to delete rows recursively (i.e. also deleting foreign key linked rows) in MySQL?

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 th开发者_Go百科at 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

1. CREATE TEMPORARY TABLES permission is required for the user running the stored procedure for the schema(s) being used.
2. MySQL only supports a maximum recursion depth of 255 so this method would fall over if there were a very large number of foreign key links (seems unlikely).
3. "Circular" / "cyclic" foreign key references (e.g. table A has a foreign key to table B and table B has a foreign key back to table A) are not currently supported and would cause an infinite loop.
4. It's not designed for use on a "live" system: Since data is deleted recursively, later deletions could fail if it happened that more data was added between deleting child and parent records.

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