Employees Earning More Than Their Managers"/>
leetcode: Employees Earning More Than Their Managers
原题如下:
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 | +----------+
本题有两种思路,一种是表的自身连接不使用join关键字,另一种是使用join关键字,第二种效率比较高。
第一种代码如下,
SELECT e1.Name from Employee e1,Employee e2where e1.ManagerId=e2.Id and e1.Salary>e2.Salary;
表名为Employee,e1和e2分别是为Employee起的别名。
第二种代码如下,
SELECT employer.NameFROM Employee employer JOIN Employee manager ON (employer.ManagerId = manager.Id )WHERE employer.Salary > manager.Salary ;
其中employer和manager也是Employee的别名,与JOIN一起使用的ON表示join的字段。
Mysql还有一个关键字left join,表示不管第二个表中有没有数据都把第一个表中的数据打印出来,leetcode的另一个题目考察了一个用法
题目连接:/
解答:
select FirstName,LastName,City,Statefrom Person left join Address on Person.PersonId=Address.PersonId
因为不存在重复,所以在select可以不用使用表名作为前缀
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leetcode: Employees Earning More Than Their Managers
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