SQL Tutorial >> SQL Foreign Key.

Now SQL Tutorial will discus SQL Foreign Key Constraint, Foreign Key Constraint is a field or fields that points to the SQL Primary Key Constraint of another table. The purpose of the SQL Foreign Key Constraint is to ensure referential integrity of the data. In other words, only those values that are supposed to appear in the database are permitted.

For example, we have two tables, a Employees table that includes all employees data, and an Employees_Salary table that includes all employees salary records. The constraint here is that all employees salaries must be associated with an employee that is already in the Employees table. In this case, we will place an SQL Foreign Key Constraint on the Employees_Salary table and have it relate to the SQL Primary Key Constraint of the Employees table. Now, we can ensure that all employees salary in the Employees_Salary table are related to a employees in the Employees table. In other words, the Employees_Salary table cannot contain information of any employee that is not in the Employees table.

The structure of these two tables will be as follows to explain the SQL Foreign Key Constraint

Structure of Employees

empID	Integer, Not Null and Primary Key
FirstName	Varchar(50)
LastName	Varchar(50)
Email	Varchar(50)
Phone	Integer

Structure of Employees_Salary

salID	Integer, Not Null
Month	Date and Time
Salary	Integer
empID	Foreign Key

In the above structure example, the empID column in the Employees_Salary table is a foreign key pointing to the empID column in the Employees table we can also change its name from empID to some others one but this convention is not recommended.  And if we use different names then the database will not generate error or did not disallow us.  But for our own acknowledgment and for our own convenience we always use the same names for the Primary key column and Foreign Key columns.

Below we show examples of how to specify the foreign key when creating the Employees_Salary table:

Below are examples for specifying an SQL Foreign Key Constraint by SQL Altering Table command. Now assume that the Employees_Salary table has been created, and the foreign key has not yet been put in.