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 | +----------+
思路一：用exists
1 SELECT Name from Employee e1 where EXISTS (SELECT ID FROM Employee e2 where e1.ManagerId = e2.Id AND e1.Salary > e2.Salary);
自连接
SELECT e1.Name FROM Employee e1 LEFT JOIN Employee e2 ON e1.ManagerId = e2.Id WHERE e1.Salary > e2.Salary;