MySQL FOREIGN KEY Constraint
MySQL FOREIGN KEY Constraint
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:
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:
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:
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:
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:
DROP FOREIGN KEY FK_PersonOrder;