MySQL外键错误：无法创建表

Question

I am trying to create two tables in MySQL database called class_members and history . The class_members table contains information about the members and is created as:

CREATE TABLE `class_members` (
  `id` int(11) NOT NULL,
  `username` varchar(65) NOT NULL DEFAULT '',
  `password` varchar(65) NOT NULL DEFAULT '',
  `email` varchar(65) NOT NULL,
  `mod_timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username_UNIQUE` (`username`),
  UNIQUE KEY `id_UNIQUE` (`id`),
  UNIQUE KEY `email_UNIQUE` (`email`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 

The class_members tables is successfully created.

I am facing the problem when creating history table which contains foreign key reference to class_members .

The command which I used is as follows:

CREATE TABLE `History` ( 
   `history_id` INT(11) NOT NULL AUTO_INCREMENT, 
   `username` VARCHAR(65) NOT NULL DEFAULT '', 
   `timestamp` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, 
   `ip` VARCHAR(20) NOT NULL,  
   PRIMARY KEY (`history_id`), 
   CONSTRAINT `fk_history_member` FOREIGN KEY (`username`) 
   REFERENCES `class_members` (`username`) 
   ON UPDATE CASCADE);

When I execute the above command I get this error:

ERROR 1005 (HY000): Can't create table 'testDB.History' (errno: 150)

I tried to find why I cannot read the History table ( about ERROR 1005 ) but I cannot understand why

---

Following Up on Comments, I created a new database and tried creating using the above command and I still get the error. See Attached Pic

---

Updated (Solved)

I found why the error in the above screenshot occurs. To solve that we have to add ENGINE=InnoDB DEFAULT CHARSET=utf8 to the end of the History table creation command.

 CREATE TABLE `History` ( 
   `history_id` INT(11) NOT NULL AUTO_INCREMENT, 
   `username` VARCHAR(65) NOT NULL DEFAULT '', 
   `timestamp` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, 
   `ip` VARCHAR(20) NOT NULL,  
   PRIMARY KEY (`history_id`), 
   CONSTRAINT `fk_history_member` FOREIGN KEY (`username`) 
   REFERENCES `class_members` (`username`) 
   ON UPDATE CASCADE)ENGINE=InnoDB DEFAULT CHARSET=utf8;

This works perfectly.

Answer 1

Your query works on rextester .

Make sure you don't already have an existing table called history . If I re-create the history table, I get:

Cannot delete or update a parent row: a foreign key constraint fails

You can reproduce this by running the rextester query once, then remove the drop table if exists history; line, and run it again.

If you try to create the table twice in the same batch, you get this instead:

Table 'history' already exists

Answer 2

I think your problem is from the origin of the index. Try the code this way;

CREATE TABLE `class_members` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `username` varchar(65) NOT NULL DEFAULT '',
  `password` varchar(65) NOT NULL DEFAULT '',
  `email` varchar(65) NOT NULL,
  `mod_timestamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`),
  UNIQUE KEY `username_UNIQUE` (`username`),
  UNIQUE KEY `id_UNIQUE` (`id`),
  UNIQUE KEY `email_UNIQUE` (`email`),
    INDEX `index_username` (`username`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `History` ( 
  `history_id` INT(11) NOT NULL AUTO_INCREMENT, 
  `username` VARCHAR(65) NOT NULL DEFAULT '', 
  `timestamp` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, 
  `ip` VARCHAR(20) NOT NULL,  
  PRIMARY KEY (`history_id`), 
  CONSTRAINT `fk_history_member` FOREIGN KEY (`username`) 
  REFERENCES `class_members` (`username`) 
  ON UPDATE CASCADE);

For SQLFiddle Sample; Click Here