Foreign Key in MySQL : ERROR 1005

Question

I'm a bit confused about adding foreign keys in MySQL

What I'm trying to do is reference the Students primary key with:

CREATE TABLE Enrolled(sid CHAR(20), cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid, cid), FOREIGN KEY (sid) REFERENCES Students);

However, what I get is

ERROR 1005 (HY000): Can't create table 'test_db.Enrolled' (errno: 150)

I searched around and found

MySQL "ERROR 1005 (HY000): Can't create table 'foo.#sql-12c_4' (errno: 150)"

However, my Students table already has a primary key so this should not be an issue:

| Students | CREATE TABLE Students (
  sid char(20) NOT NULL DEFAULT '',
  name char(20) DEFAULT NULL,
  login char(10) DEFAULT NULL,
  age int(11) DEFAULT NULL,
  gpa float DEFAULT NULL,
  PRIMARY KEY (sid)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

I feel like I'm missing something fairly basic but I can't appear to spot it. Any advice?

Answer 1

What about making the referenced column explicit? Something like:

CREATE TABLE Enrolled(
     sid CHAR(20), 
     cid CHAR(20), 
     grade CHAR(2), 

     PRIMARY KEY (sid, cid), 
     FOREIGN KEY (sid) REFERENCES Students (sid)
);

Look at FOREIGN KEY (sid) REFERENCES Students (sid), last line. Also note that if the foreign key is a single column you can specify it just after the column type:

CREATE TABLE Enrolled(
    sid CHAR(20) REFERENCES Students (sid), 
    ...

Answer 2

You should add referenced column name after referenced table REFERENCES Students(sid). So you have to change your code to below solution

CREATE TABLE Enrolled
(
  sid CHAR(20), 
  cid CHAR(20), 
  grade CHAR(2), 
  PRIMARY KEY (sid, cid), 
  FOREIGN KEY (sid) REFERENCES Students(sid)
);