Why cant i add this foreign key constraint? - mysql

Question

Im frustrated. I cannot add the foreign key constraint for the table Test_info:
FOREIGN KEY Test_info(score) REFERENCES Test(score)
It keeps on giving me error 1215 Cannot add foreign key constraint. I made sure the data type is the same, as well as names etc... anyone?
The entire SQL Table creation is:
CREATE TABLE IF NOT EXISTS Employees (
ssn VARCHAR(20),
union_mem_no VARCHAR(20),
PRIMARY KEY (ssn)
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Traffic_Control (
ssn VARCHAR(20),
exam_date DATE,
PRIMARY KEY (ssn),
FOREIGN KEY (ssn) REFERENCES Employees(ssn) ON DELETE CASCADE
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Technician (
ssn VARCHAR(20),
T_name VARCHAR(20),
phone_num VARCHAR(20),
address VARCHAR(50),
Salary INT,
PRIMARY KEY (ssn),
FOREIGN KEY (ssn) REFERENCES Employees(ssn) ON DELETE CASCADE
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Model (
model_no VARCHAR(20),
Capacity INT,
Weight INT,
PRIMARY KEY (model_no)
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Expert (
ssn VARCHAR(20),
model_no VARCHAR(20),
FOREIGN KEY (ssn) REFERENCES Technician(ssn) ON DELETE CASCADE,
FOREIGN KEY (model_no) REFERENCES Model(model_no) ON DELETE CASCADE
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Plane (
reg_no VARCHAR(20),
PRIMARY KEY (reg_no)
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Type (
reg_no VARCHAR(20),
model_no VARCHAR(20),
FOREIGN KEY (reg_no) REFERENCES Plane(reg_no),
FOREIGN KEY (model_no) REFERENCES Model(model_no)
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Test (
FAA_no VARCHAR(20),
T_name VARCHAR(20),
score INT,
PRIMARY KEY (FAA_no)
) ENGINE=INNODB;
CREATE TABLE IF NOT EXISTS Test_info (
ssn VARCHAR(20),
FAA_no VARCHAR(20),
T_date DATE NOT NULL,
hours INT,
score INT,
PRIMARY KEY (ssn, FAA_no),
FOREIGN KEY (ssn) REFERENCES Employees(ssn) ON DELETE CASCADE,
FOREIGN KEY (FAA_no) REFERENCES Test(FAA_no),
FOREIGN KEY (score) REFERENCES Test(score)
) ENGINE=INNODB;

Answer 1

You have to define a UNIQUE constraint on the column you're referencing to be able to create a foreign key relation to that column.
However, from your data schema it looks like the score column would probably not have unique values.
Since you already established a foreign key relation with the Test table's primary key (FAA_no), I don't see the point of adding a second foreign key.