SQL error 1215(HY000) cannot add foreign key constraint. - mysql

Question

I looked through other questions on this topic, and the general consensus was to check my data type consistency. I am trying to create the table 'reviews' that has SSN and ProjectID as foreign keys, referencing SSN from Table Reviewer and ProjectID from Table Project
CREATE TABLE Reviews(
ProjectID VARCHAR(10) NOT NULL,
SSN INTEGER NOT NULL CHECK
(SSN>100000000 AND SSN<999999999),
ReviewerRole VARCHAR(50) NOT NULL,
FOREIGN KEY(SSN)REFERENCES Reviewer(SSN),
FOREIGN KEY(ProjectID)REFERENCES Project(ProjectID),
PRIMARY KEY(SSN,ProjectID)
);
The following are the create table statements for 'Project' and 'Reviewer'
CREATE TABLE Reviewer(
SSN INTEGER NOT NULL CHECK (SSN>100000000 AND SSN<999999999),
Firstname VARCHAR(50) NOT NULL,
Lastname VARCHAR(50) NOT NULL
);
CREATE TABLE Project(
ProjectID VARCHAR(10) NOT NULL,
Title VARCHAR(50),
Archived DATE NOT NULL,
ProjectStatus VARCHAR(50) NOT NULL,
PRIMARY KEY(ProjectID)
);
Project and Reviewer created without issue. My create Reviews query works if I remove the FK declaration for SSN and leave the FK declaration for ProjectID, but not the other way around.
What is the reason for the error? Is there another constraint I need to add?

Answer 1

You forgot to add a primary key to the SSN column in the Reviewer table. The query below builds correctly in sqlfiddle:
CREATE TABLE Reviewer(
SSN INTEGER NOT NULL CHECK (SSN>100000000 AND SSN<999999999),
Firstname VARCHAR(50) NOT NULL,
Lastname VARCHAR(50) NOT NULL,
PRIMARY KEY(SSN)
);
CREATE TABLE Project(
ProjectID VARCHAR(10) NOT NULL,
Title VARCHAR(50),
Archived DATE NOT NULL,
ProjectStatus VARCHAR(50) NOT NULL,
PRIMARY KEY(ProjectID)
);
CREATE TABLE Reviews(
ProjectID VARCHAR(10) NOT NULL,
SSN INTEGER NOT NULL CHECK
(SSN>100000000 AND SSN<999999999),
ReviewerRole VARCHAR(50) NOT NULL,
FOREIGN KEY(SSN)REFERENCES Reviewer(SSN),
FOREIGN KEY(ProjectID)REFERENCES Project(ProjectID),
PRIMARY KEY(SSN,ProjectID)
);

Answer 2

A foreign key can only reference to a Primary Key column in another table, you need to make SNN a primary key column.
CREATE TABLE Reviewer(
SSN INTEGER NOT NULL primary key CHECK (SSN>100000000 AND SSN<999999999),
Firstname VARCHAR(50) NOT NULL,
Lastname VARCHAR(50) NOT NULL
);

Answer 3

The referenced column for a foreign key column, needs to be Primary key OR have a unique constraint so your Reviewer table need to be:
CREATE TABLE Reviewer(
SSN INTEGER PRIMARY KEY CHECK (SSN>100000000 AND SSN<999999999),
Firstname VARCHAR(50) NOT NULL,
Lastname VARCHAR(50) NOT NULL
);
or
CREATE TABLE Reviewer(
SSN INTEGER NOT NULL CHECK (SSN>100000000 AND SSN<999999999),
Firstname VARCHAR(50) NOT NULL,
Lastname VARCHAR(50) NOT NULL,
UNIQUE(SSN)
);
Also as far as I know, MySQL Ignores Check Constraints, so you can remove them.