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:
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;