Question

Refer: w3schools-SQL

It states:

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. Can someone give an example of an action that foreign key would prevent ?

Thanks.

Answer 1

Lets say we have the two tables Employees and Departments. Each department has a unique number, and for each employee we list the number of the department that he or she works on. We have also declared this as a foreign key.

In that case the foreign key would prevent a number of actions:

• Inserting a new employee who works on a department that doesn’t exist
• Deleting a department where there are employees who work
• Updating an employee by changing the department he or she works on to a department that doesn’t exist
• Updating a department by changing its number, if there are employees who work on that department
• Destroying the department table

All this is to ensure that when the database says that an employee works on a certain department, then that department will actually exist in the database.