How to fix ERROR 1217: Cannot delete or update a parent row: a foreign key constraint fails in MySQL

-- Find child records referencing the parent
SELECT * FROM child_table WHERE parent_id = 123;

-- Delete child records that reference the parent
DELETE FROM child_table WHERE parent_id = 123;

-- Now delete the parent row
DELETE FROM parent_table WHERE id = 123;

-- First, find the constraint name
SHOW CREATE TABLE child_table;

-- Drop the old constraint
ALTER TABLE child_table DROP FOREIGN KEY fk_old_constraint_name;

-- Recreate with CASCADE options
ALTER TABLE child_table ADD CONSTRAINT fk_parent_id 
  FOREIGN KEY (parent_id) REFERENCES parent_table(id)
  ON DELETE CASCADE
  ON UPDATE CASCADE;

-- Drop the old constraint
ALTER TABLE child_table DROP FOREIGN KEY fk_old_constraint_name;

-- Recreate with SET NULL option
ALTER TABLE child_table ADD CONSTRAINT fk_parent_id 
  FOREIGN KEY (parent_id) REFERENCES parent_table(id)
  ON DELETE SET NULL
  ON UPDATE SET NULL;

ALTER TABLE child_table MODIFY parent_id INT NULL;

-- Test deletion of parent row
DELETE FROM parent_table WHERE id = 123;

-- Verify it worked
SELECT COUNT(*) FROM parent_table WHERE id = 123;

SELECT COUNT(*) FROM child_table WHERE parent_id = 123;