Constraints

 

Now that we know how to insert, update and delete data from our tables, let’s revisit some of the constraints that we placed on our tables when we created them.

 

Remember that we have two foreign key constraints for the mentorships table? Let’s see how they work.

 

Try inserting the following data into mentorships:

 

INSERT INTO mentorships VALUES

(4, 21, 'Ongoing', 'Flynn Tech');

 

What happens? You get an error message that says

 

Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails

 

right? This is due to the fact that we do not have an employee with id = 21 in the employees table. Hence, we cannot insert a row with mentee_id = 21 into the mentorships table as the mentee_id column is supposed to reference the id column in the employees table.

 

Next, let’s try to update some data in the employees table. For instance, try adding the code below...