具有多个表的SQL Join查询

Question

I have a database with the following layout:

employees (person_name, street, city) 
works (person_name, company_name, salary) 
companies (company_name, city) 
manages (person_name, manager_name)

I need to come up with a query that will allow me to find all employees who are earning more then their managers. I know this involves some pretty intense JOINing between the employees, works, and manages tables, and I can't quite get one that will work. Can someone help me out, and explain how the solution is obtained?

Answer 1

First off, name isn't a great key.

The key to the answer is you can join to a table more than once (remember to alias your tables so you can distinguish which one you want to reference).

Without writing the query for you it means you should select Employees, join through Works and Companies (this gives you employee salary). This should be joined through Manages which joins back to Employees to get your manager details. This version of Employees can be joined to Works for the second time to get your manager salary. Then just compare the salaries in your WHERE clause.

Answer 2

SELECT w.person_name
FROM works w 
    INNER JOIN manages m
        ON m.person_name = w.person_name
            INNER JOIN works wManager
                ON wManager.person_name = m.manager_name
WHERE w.salary > wManager.salary