无法在表中添加两个外键

Question

I have two tables as follow: 1st Table:

CREATE TABLE User ( 
            User_ID VARCHAR(8)NOT NULL PRIMARY KEY,
            User_Name VARCHAR (25) NOT NULL,
            User_Gender CHAR (1) NOT NULL,
            User_Position VARCHAR (10) NOT NULL,
          );

2nd table:

CREATE TABLE Training (
            Training_Code VARCHAR(8) NOT NULL Primary Key,
            Training_Title VARCHAR(30) NOT NULL,
            );

I am trying to create a table which has two foreign keys to join both of the previous tables:

CREATE TABLE Request (
            User_ID VARCHAR(8) NOT NULL, 
            Training_Code VARCHAR(8) NOT NULL, 
            Request_Status INT(1) NOT NULL
            );

When I am trying to set the foreign keys in the new table, the User_ID can be done successfully but the Training_Code cannot be set to foreign key due to the error:

ERROR 1215 (HY000): Cannot add foreign key constraint

As I searched for this problem, the reason for it, is that data type is not the same, or name is not the same.. but in my situation both are correct so could you tell me what is wrong here ?

Answer 1

You need an index for this column in table Request too:

Issue first

CREATE INDEX idx_training_code ON Request (Training_Code);

Then you should be successful creating the foreign key constraint with

ALTER TABLE Request
ADD CONSTRAINT FOREIGN KEY idx_training_code (Training_Code) 
    REFERENCES Training(Training_Code);

It worked for me. But I've got to say that it worked without create index too, as the documentation of Using FOREIGN KEY Constraints states:

MySQL requires indexes on foreign keys and referenced keys so that foreign key checks can be fast and not require a table scan. In the referencing table, there must be an index where the foreign key columns are listed as the first columns in the same order. Such an index is created on the referencing table automatically if it does not exist. This index might be silently dropped later, if you create another index that can be used to enforce the foreign key constraint. index_name, if given, is used as described previously.

Emphasis by me. I don't know what's the issue in your case.

Demo

Explanation of the issue

The behavior mentioned in the question can be reproduced if the table Training is using the MyISAM storage engine. Then creating a foreign key referencing the table Training will produce the mentioned error.

If there's data in the table, then simple dropping of the table would not be the best solution. You can change the storage engine to InnoDB with

ALTER TABLE Training Engine=InnoDB;

Now you can successfully add the foreign key constraint.