Question

The Employee table holds all employees including their managers. Every employee has an Id, and there is also a column for the manager Id.

+----+-------+--------+-----------+
| Id | Name  | Salary | ManagerId |
+----+-------+--------+-----------+
| 1  | Joe   | 70000  | 3         |
| 2  | Henry | 80000  | 4         |
| 3  | Sam   | 60000  | NULL      |
| 4  | Max   | 90000  | NULL      |
+----+-------+--------+-----------+

Given the Employee table, write a SQL query that finds out employees who earn more than their managers. For the above table, Joe is the only employee who earns more than his manager.

+----------+
| Employee |
+----------+
| Joe      |
+----------+

SQL Schema

Create table If Not Exists Employee (Id int, Name varchar(255), Salary int, ManagerId int)
Truncate table Employee
insert into Employee (Id, Name, Salary, ManagerId) values ('1', 'Joe', '70000', '3')
insert into Employee (Id, Name, Salary, ManagerId) values ('2', 'Henry', '80000', '4')
insert into Employee (Id, Name, Salary, ManagerId) values ('3', 'Sam', '60000', 'None')
insert into Employee (Id, Name, Salary, ManagerId) values ('4', 'Max', '90000', 'None')

My Interesting Code

select origin.Name as Employee
from Employee as origin
inner join Employee as temp
on origin.ManagerId = temp.Id
where origin.Salary > temp.Salary

My Perspective

For this question, because you only have one table, you can use “inner join” to combine the same table twice accoring to the “MangerId” and “Id”. Then you can just compare the value of “Salary” is ok.

More importantly, the new table contains all the keys twice, so I suggest you can use “as” to distinguish these two tables, which can make “select” more conveniently.

Also, there is a useful links, and I think it can help you. (chinese version)