mysql 1215无法添加外键约束

Question

I have a SQL script that yields me the error:

DROP TABLE IF EXISTS test_db.users
;

CREATE TABLE users
(
  id SERIAL, 
  username VARCHAR(20) NOT NULL,
  password VARCHAR(20) NOT NULL,
  PRIMARY KEY (id)
);

DROP TABLE IF EXISTS test_db.comments
;

CREATE TABLE comments
(
  id SERIAL,
  content varchar(255) NOT NULL,
  userId BIGINT(20) NOT NULL,
  CONSTRAINT fk_comments_has_user
  FOREIGN KEY (userId)
  REFERENCES test_db.users(id) 
  ON DELETE CASCADE,
  PRIMARY KEY (id)
);

ERROR 1215 (HY000): Cannot add foreign key constraint

This error is not so specific, and I can't really seem to pinpoint the error by reading other posts regarding similar error.

Answer 1

The datatypes need to be the same bigint is not the same as serial. Try this

drop table if exists comments;
DROP TABLE IF EXISTS temp;

CREATE TABLE temp
(
  id bigint auto_increment, 
  username VARCHAR(20) NOT NULL,
  password VARCHAR(20) NOT NULL,
  PRIMARY KEY (id)
);

DROP TABLE IF EXISTS comments
;

CREATE TABLE comments
(
  id bigint auto_increment,
  content varchar(255) NOT NULL,
  userId bigint NOT NULL,
  CONSTRAINT fk_comments_has_user
  FOREIGN KEY (userId)
  REFERENCES temp(id) 
  ON DELETE CASCADE,
  PRIMARY KEY (id)
);