A primary key is a field or group of fields that uniquely identify a record in a table. Primary key fields cannot be NULL and cannot contain duplicate values.
If you want to link two tables, then primary key of one table will be added to another table where primary key of first table will be become the foreign key of second table.
Consider the following two tables.
|Department_ID (Primary Key)||Department_Name|
|Employee_ID (Primary Key)||Name||Place|
If you want to link department table and employee table, then add the primary key of department table i.e. Department_ID to employee table. Department_ID becomes the foreign key of employee table and Department table becomes the check table.
Employee table after creating foreign key.
|Employee_ID (Primary Key)||Name||Place||Department_ID (Foreign Key)|
The main purpose of the foreign key is data validation. This will not allow entering a Department_ID in the employee table that is not there in the department table. For example if you try to create a record in employee table with Department_ID as ’04’, it will throw an error.