[leetcode/database] 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      |
+----------+

Using self join:

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

The SQL SELF JOIN is used to join a table to itself as if the table were two tables.
The basic syntax of SELF JOIN is as follows:

SELECT a.column_name, b.column_name…
FROM table1 a, table1 b
WHERE a.common_field = b.common_field;

As this table has the employee’s manager information, we need to select information from it twice. Though we can also solve the problem with WHERE & AND/OR, JOIN & ON is a more common and efficient way to link tables together.