Question

This may seems the most dumbest question ever on stackoverflow but I am just wondering why would you write such a query:

Select e1.Emploee_ID, e1.Departement_ID From Employee e 
Inner join Employee E1 on e.employee_id= e1.Employee_ID
Inner join Departement d on d.dep_id= e1.departement_id

Why do we have to join on employee? my obvious query would be

select e.employee_ID, e.Departement_id from employee e 
inner join  Departement d on d.dep_id= e1.departement_id

Answer 1

Referencing the PK with an inner join is redundant.

You would normally join on the same table to link with another record, for example if you have a FK-column that reference the boss of an employee.

Assuming you would have a nullable foreign-key column Boss_ID in table Employee

Select e.Employee_ID, boss.Employee_id, d.Departement_ID 
From Employee e  
LEFT OUTER JOIN Employee boss on boss.Employee_ID=e.Boss_ID
INNER JOIN Departement d on d.dep_id= e.departement_id

Note that i’ve used a LEFT OUTER JOIN to get also the employee that have no bosses.