题目描述
Employee 表包含所有员工,他们的经理也属于员工。每个员工都有一个 Id,此外还有一列对应员工的经理的 Id。
+----+-------+--------+-----------+
| Id | Name | Salary | ManagerId |
+----+-------+--------+-----------+
| 1 | Joe | 70000 | 3 |
| 2 | Henry | 80000 | 4 |
| 3 | Sam | 60000 | NULL |
| 4 | Max | 90000 | NULL |
+----+-------+--------+-----------+
给定 Employee 表,编写一个 SQL 查询,该查询可以获取收入超过他们经理的员工的姓名。在上面的表格中,Joe 是唯一一个收入超过他的经理的员工。
+----------+
| Employee |
+----------+
| Joe |
+----------+
解法一:子查询
SELECT
Name Employee
FROM
Employee AS a
WHERE
Salary > (SELECT
Salary
FROM
Employee
WHERE
Id = a.Managerid)
解法二:给表另起别名重用
select e1.Name as Employee from Employee as e1 ,Employee as e2 where e1.ManagerId=e2.Id AND e1.Salary>e2.Salary
结论:由于解法一使用了子查询,所以执行效率上会消耗更多的资源和时间,解法二想比解法一效率高上许多