Foreign Key Constraint in SQL

The FOREIGN KEY is used to define a relationship between two tables and a FOREIGN KEY in one table points to the PRIMARY KEY in another table.

Syntax of FOREIGN KEY Constraint on one column with CREATE TABLE statement:

MySQL:

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

SQL Server / Oracle / MS Access:

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

Syntax of FOREIGN KEY Constraint on one column with ALTER TABLE statement:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders DROP FOREIGN KEY fk_POrders

Syntax of FOREIGN KEY Constraint on one column with CREATE TABLE statement:

MySQL / SQL Server / Oracle / MS Access:

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

Syntax of FOREIGN KEY Constraint on one column with ALTER TABLE statement:

MySQL / SQL Server / Oracle / MS Access:

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

DropFOREIGNKEY:

Use the following syntax to delete the foreign key.

MySQL:

ALTER TABLE Orders DROP FOREIGN KEY fk_POrders

SQL Server / Oracle / MS Access:

ALTER TABLE Orders DROP CONSTRAINT fk_POrders

Difference between primary key and foreign key:

Primary key

• Primary key can’t contain null values.
• Primary key can’t be duplicate.
• A table can have only one primary key.
• Primary key automatically adds a clustered index.

Foreign key

• Foreign key can contain null values.
• Foreign key can be duplicate.
• A table can have more than one foreign key.
• Foreign key not add any index automatically.