181. Employees Earning More Than Their Managers


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      |
+----------+

Solution

Approach I: Using WHERE clause [Accepted]

Algorithm

As this table has the employee's manager information, we probably need to select information from it twice.

SELECT *
FROM Employee AS a, Employee AS b
;

Note: The keyword 'AS' is optional.

Id Name Salary ManagerId Id Name Salary ManagerId
1 Joe 70000 3 1 Joe 70000 3
2 Henry 80000 4 1 Joe 70000 3
3 Sam 60000 1 Joe 70000 3
4 Max 90000 1 Joe 70000 3
1 Joe 70000 3 2 Henry 80000 4
2 Henry 80000 4 2 Henry 80000 4
3 Sam 60000 2 Henry 80000 4
4 Max 90000 2 Henry 80000 4
1 Joe 70000 3 3 Sam 60000
2 Henry 80000 4 3 Sam 60000
3 Sam 60000 3 Sam 60000
4 Max 90000 3 Sam 60000
1 Joe 70000 3 4 Max 90000
2 Henry 80000 4 4 Max 90000
3 Sam 60000 4 Max 90000
4 Max 90000 4 Max 90000
> The first 3 columns are from a and the last 3 ones are from b.

Select from two tables will get the Cartesian product of these two tables. In this case, the output will be 4*4 = 16 records. However, what we interest is the employee's salary higher than his/her manager. So we should add two conditions in a WHERE clause like below.

SELECT
    *
FROM
    Employee AS a,
    Employee AS b
WHERE
    a.ManagerId = b.Id
        AND a.Salary > b.Salary
;
Id Name Salary ManagerId Id Name Salary ManagerId
1 Joe 70000 3 3 Sam 60000

As we only need to output the employee's name, so we modify the above code a little to get a solution.

MySQL

SELECT
    a.Name AS 'Employee'
FROM
    Employee AS a,
    Employee AS b
WHERE
    a.ManagerId = b.Id
        AND a.Salary > b.Salary
;

Approach I: Using JOIN clause [Accepted]

Algorithm

Actually, JOIN is a more common and efficient way to link tables together, and we can use ON to specify some conditions.

SELECT
     a.NAME AS Employee
FROM Employee AS a JOIN Employee AS b
     ON a.ManagerId = b.Id
     AND a.Salary > b.Salary
;
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