MySQL : ERROR 1215 (HY000): Cannot add foreign key constraint

Question

I have read Database system concepts, 6th edition, Silberschatz. I\'m going to implement the university database system shown in chapter 2 on OS X

Answer 1

The syntax of FOREIGN KEY for CREATE TABLE is structured as follows:

FOREIGN KEY (index_col_name)
        REFERENCES table_name (index_col_name,...)

So your MySQL DDL should be:

 create table course (
        course_id varchar(7),
        title varchar(50),
        dept_name varchar(20),
        credits numeric(2 , 0 ),
        primary key (course_id),
        FOREIGN KEY (dept_name)
            REFERENCES department (dept_name)
    );

Also, in the department table dept_name should be VARCHAR(20)

More information can be found in the MySQL documentation

Answer 2

Maybe your dept_name columns have different charsets.

You could try to alter one or both of them:

ALTER TABLE department MODIFY dept_name VARCHAR(20) CHARACTER SET utf8;
ALTER TABLE course MODIFY dept_name VARCHAR(20) CHARACTER SET utf8;

Answer 3

It's worth noting that this error can also happen if the target table or column you're using in the REFERENCES portion simply doesn't exist.

Answer 4

It is also possible to get this error if the foreign key is not a primary key within its own table.

I did an ALTER TABLE and accidentally removed the primary key status of a column, and got this error.