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

A FOREIGN KEY is a field (or collection of fields) in one table, that refers to
the PRIMARY KEY in another table.

The table with the foreign key is called the child table, and the table
with the primary key is called the referenced or parent table.

Look at the following two tables:

Persons Table

PersonID LastName FirstName Age
1 Hansen Ola 30
2 Svendson Tove 23
3 Pettersen Kari 20

Orders Table

OrderID OrderNumber PersonID
1 77895 3
2 44678 3
3 22456 2
4 24562 1

Notice that the “PersonID” column in the “Orders” table points to the “PersonID” column in the “Persons” table.

The “PersonID” column in the “Persons” table is the PRIMARY KEY in the “Persons” table.

The “PersonID” column in the “Orders” table is a
FOREIGN KEY in the “Orders” table.

The FOREIGN KEY constraint prevents invalid data from being inserted into the foreign key column,
because it has to be one of the values contained in the parent table.

FOREIGN KEY on CREATE TABLE

The following SQL creates a FOREIGN KEY on the “PersonID” column when the “Orders” table is created:

CREATE TABLE Orders
(

OrderID int NOT NULL,

OrderNumber int NOT NULL,

PersonID int,

PRIMARY KEY (OrderID),

FOREIGN KEY (PersonID) REFERENCES Persons(PersonID)
);





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

CREATE TABLE Orders
(

OrderID int NOT NULL,

OrderNumber int NOT NULL,

PersonID int,

PRIMARY KEY (OrderID),

CONSTRAINT FK_PersonOrder FOREIGN KEY (PersonID)

REFERENCES Persons(PersonID)
);






FOREIGN KEY on ALTER TABLE

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

ALTER TABLE Orders
ADD FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID);

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

ALTER TABLE Orders
ADD CONSTRAINT FK_PersonOrder
FOREIGN KEY (PersonID)
REFERENCES Persons(PersonID);

DROP a FOREIGN KEY Constraint

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

ALTER TABLE Orders
DROP FOREIGN KEY FK_PersonOrder;