SQL Foreign Keys Foreign Keys explained
Example
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).
