SQL FOREIGN KEY Constraint

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# SQL FOREIGN KEY Constraint

A FOREIGN KEY in one table points to a PRIMARY KEY in another table.

Let's illustrate the foreign key with an example. Look at the following two tables:

The "Persons" table:

P_Id | LastName | FirstName | Address | City
--- | --- | --- | --- | ---
1 | Hansen | Ola | Timoteivn 10 | Sandnes
2 | Svendson | Tove | Borgvn 23 | Sandnes
3 | Pettersen | Kari | Storgt 20 | Stavanger

The "Orders" table:

O_Id | OrderNo | P_Id
--- | --- | ---
1 | 77895 | 3
2 | 44678 | 3
3 | 22456 | 2
4 | 24562 | 1

Note that the "P_Id" column in the "Orders" table points to the "P_Id" column in the "Persons" table.

The "P_Id" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.

The "P_Id" column in the "Orders" table is a FOREIGN KEY in the "Orders" table.

The FOREIGN KEY constraint is used to prevent actions that would destroy link between tables.

The FOREIGN KEY constraint also prevents that invalid data is inserted into the foreign key column, because it has to be one of the values contained in the table it points to.

## SQL FOREIGN KEY Constraint on CREATE TABLE

The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created:

MySQL:

    CREATE TABLE Orders
    (
    O_Id int NOT NULL,
    OrderNo int NOT NULL,
    P_Id int,
    PRIMARY KEY (O_Id),
    FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
    )

SQL Server / Oracle / MS Access:

    CREATE TABLE Orders
    (
    O_Id int NOT NULL PRIMARY KEY,
    OrderNo int NOT NULL,
    P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
    )

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

    CREATE TABLE Orders
    (
    O_Id int NOT NULL,
    OrderNo int NOT NULL,
    P_Id int,
    PRIMARY KEY (O_Id),
    CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
    REFERENCES Persons(P_Id)
    )

## SQL FOREIGN KEY Constraint on ALTER TABLE

To create a FOREIGN KEY constraint on the "P_Id" column when the "Orders" table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

    ALTER TABLE Orders
    ADD FOREIGN KEY (P_Id)
    REFERENCES Persons(P_Id)

To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

    ALTER TABLE Orders
    ADD CONSTRAINT fk_PerOrders
    FOREIGN KEY (P_Id)
    REFERENCES Persons(P_Id)

## To DROP a FOREIGN KEY Constraint

To drop a FOREIGN KEY constraint, use the following SQL:

MySQL:

    ALTER TABLE Orders
    DROP FOREIGN KEY fk_PerOrders

SQL Server / Oracle / MS Access:

    ALTER TABLE Orders
    DROP CONSTRAINT fk_PerOrders

From http://www.w3schools.com (Copyright Refsnes Data)