Foreign Keys constraints ensure data integrity, by enforcing that values in one table must match values in another table.

An example of where a foreign key is required is: In a university, a course must belong to a department. Code for the this scenario is:

CREATE TABLE Department (
    Dept_Code        CHAR (5)     PRIMARY KEY,
    Dept_Name        VARCHAR (20) UNIQUE
);

Insert values with the following statement:

INSERT INTO Department VALUES ('CS205', 'Computer Science');

The following table will contain the information of the subjects offered by the Computer science branch:

CREATE TABLE Programming_Courses (
    Dept_Code       CHAR(5),
    Prg_Code        CHAR(9) PRIMARY KEY,
    Prg_Name        VARCHAR (50) UNIQUE,
    FOREIGN KEY (Dept_Code) References Department(Dept_Code)
);

(The data type of the Foreign Key must match the datatype of the referenced key.)

The Foreign Key constraint on the column Dept_Code allows values only if they already exist in the referenced table, Department. This means that if you try to insert the following values:

INSERT INTO Programming_Courses Values ('CS300', 'FDB-DB001', 'Database Systems');

the database will raise a Foreign Key violation error, because CS300 does not exist in the Department table. But when you try a key value that exists:

INSERT INTO Programming_Courses VALUES ('CS205', 'FDB-DB001', 'Database Systems');
INSERT INTO Programming_Courses VALUES ('CS205', 'DB2-DB002', 'Database Systems II');

then the database allows these values.

A few tips for using Foreign Keys

• A Foreign Key must reference a UNIQUE (or PRIMARY) key in the parent table.
• Entering a NULL value in a Foreign Key column does not raise an error.
• Foreign Key constraints can reference tables within the same database.
• Foreign Key constraints can refer to another column in the same table (self-reference).