Foreign Key Constraint
SQL Foreign Key Constraint
A SQL Foreign Key constraint is a column or combination of columns that is used to create link between the data in two tables.
When a value other than NULL is entered into the column of a FOREIGN KEY constraint, the value must exist in the referenced column.
FOREIGN KEY constraints can reference only tables within the same database on the same server.
Example
CREATE TABLE Test1 ( ID INT PRIMARY KEY, TITLE VARCHAR(250) NOT NULL, ); CREATE TABLE Test2 ( ID INT PRIMARY KEY, TITLE VARCHAR(250) NOT NULL, CONSTRAINT FK_Test1 FOREIGN KEY (ID) REFERENCES Test1 (ID) ON DELETE CASCADE ON UPDATE CASCADE ); CREATE TABLE Test3 ( ID INT PRIMARY KEY, ID_TEST INT NOT NULL, CONSTRAINT UK_IDTEST UNIQUE(ID_TEST) ); ALTER TABLE Test3 ADD CONSTRAINT FK_Test13 FOREIGN KEY (ID_TEST) REFERENCES Test1 (ID) ;