FOREIGN KEY Constraint
The Foreign Key Constraints is used to link two tables together.
The foreign key from one table is linked to the primary key from another table.
SQL FOREIGN KEY ON CREATE TABLE
Syntax
CREATE TABLE <TABLE_NAME>(
<COLUMN_NAME> <DATATYPE>,
<COLUMN_NAME> <DATATYPE>,
<COLUMN_NAME> <DATATYPE> FOREIGN KEY REFERENCES <MASTER_TABLE>(<COLUMN_NAME>)
);
Example
CREATE TABLE Employee(
Emp_id Integer PRIMARY KEY,
Emp_Name Varchar(20),
Emp_Gender Varchar(10),
Dept_Id Integer FOREIGN KEY REFERENCES Department(Dept_Id)
);
SQL FOREIGN KEY ON ALTER TABLE
Syntax
ALTER TABLE <TABLE_NAME>
ADD FOREIGN KEY (<COLUMN_NAME>) REFERENCES <MASTER_TABLE> (<COLUMN_NAME>);
Example
ALTER TABLE Employee
ADD FOREIGN KEY (Dept_Id) REFERENCES Department(Dept_Id);
Summary
The Foreign Key Constraints is used to link two tables together.