Error 1215: Why is it happening? - mysql

Question

I am confused as to why error 1215: cannot add foreign key constraint is occurring in my code. Does anybody have any ideas?
The first four tables create themselves fine, but the error is thrown when I try to create the Stars table. I am not sure what's going wrong, as the keys being referenced are primary keys and so they must be unique and non-null. Plus, those keys exist as the previous tables have been created first. It might just be a stupid mistake I made, but its better to have fresh eyes look at it, right?
This database is simplistic as it is because I'm doing it for a school assignment.
create table MovieExec
(
execName varchar(40),
certNum numeric(30, 0),
address varchar(50),
networth real,
primary key(execName),
unique key(certNum)
);
create table Stud
(
studName varchar(30),
address varchar(50),
presCNum numeric(30, 0),
primary key(studName),
foreign key (presCNum) references MovieExec(certNum)
);
create table MovieStar(
starName varchar(30),
address varchar(50),
gender varchar(1),
birthdate date,
primary key(starName)
);
create table Movies
(
movieTitle varchar(30),
movieYear numeric(4,0),
length numeric(3,0),
genre varchar(30),
studioName varchar(30),
producerCNum numeric(30, 0),
primary key (movieTitle, movieYear),
foreign key (producerCNum) references MovieExec(certNum),
foreign key (studioName) references Stud(studName)
);
create table Stars
(
movieTitle varchar(30),
movieYear numeric(4,0),
starName varchar(30),
primary key (movieTitle, movieYear, starName),
foreign key (movieTitle) references Movies(movieTitle),
foreign key (movieYear) references Movies(movieYear),
foreign key (starName) references MovieStar(starName)
);

Answer 1

It should be a composite foreign key for the title and the year:
foreign key (movieTitle, movieYear) references Movies(movieTitle, movieYear),
foreign key (starName) references MovieStar(starName)
It should be composite because that's the exact PK of Movies table and that's the only UNIQUE combination there currently available.
Then as #CBroe mentioned in their answer it would be a good idea to index the Stars.starName column.

Answer 2

http://dev.mysql.com/doc/refman/5.5/en/innodb-foreign-key-constraints.html:
“[…] in the referenced table, there must be an index where the referenced columns are listed as the first columns in the same order”
In your Movies table, you only have a primary key on the combination (movieTitle, movieYear) – but movieYear is not the first column in that key, and therefor
foreign key (movieYear) references Movies(movieYear)
in your CREATE statement for the stars table fails.
Add a key on Movies.movieYear – then creating a foreign key referencing that column on the stars table will work.
FYI: MySQL has a YEAR data type – you should use that, instead of a NUMERIC for your movie year.
And you are mixing singular and plural in your table names. Convention is to use the singular of the object that a table holds as table name; but at least you should try and be consistent with your naming scheme.