学习 SQL：需要帮助 使用 HAVING 从聚合中获取 MIN 结果

Question

希望能得到一些帮助。刚接触 SQL 并试图保持头脑清醒。

我接到了这个任务：

“使用 HAVING，确定哪些直接下属的平均工资最低。”

我的逻辑是在一个派生表上进行连接，该表将计算每个拥有直接下属的经理的船员平均工资。然后，此结果将通过 HAVING 子句过滤为 MIN 值。

SELECT Flash.firstname, Flash.lastname, Wally.AVGSalary, 
       Wally.[Direct Reports] FROM CrewMembers Flash
    JOIN
    (
        SELECT Barry.crewMemberId, AVG(Zoom.salary) AS 'AVGSalary', COUNT(Zoom.firstname) AS 'Direct Reports' 
                    FROM CrewMembers Barry
            JOIN CrewMembers Zoom 
                           ON Zoom.managerId = Barry.crewMemberId
        GROUP BY Barry.crewMemberId 
    ) 
    AS Wally ON Wally.crewMemberId = Flash.crewMemberId

GROUP BY Flash.firstname, Flash.lastname, Wally.AVGSalary, Wally.[Direct Reports]

HAVING MIN(Wally.AVGSalary) in (Wally.AVGSalary)

ORDER BY Wally.AVGSalary asc 

我得到了这个结果：

firstname   lastname    AVGSalary   Direct Reports
Mike        Patton      33666.500000    2
Kurt        Corgan      37300.000000    2
Amber       Bruckner    45851.666666    3
Doug        Adams       86250.000000    2
Montgomery  Scott       92500.000000    2
James       Kirk        132666.750000   3

我被难住了。我需要它给我只有最小值的行，它给了我每一个值。我一直盯着这个，不知道我做错了什么。

我知道，从在线查看线程来看，使用 TOP 是一种过滤项目的方法，但尚未引入该方法，我希望保持在迄今为止课程中已建立的参数范围内。

任何对我所写内容的批评或任何帮助都会很棒。就像我说的，我是新手，只是想跟上。

谢谢，

J

Answer 1

您需要另一个子查询（如果您不想使用顶部/窗口功能/等）：

select Flash.firstname,
    Flash.lastname,
    Wally.AVGSalary,
    Wally.[Direct Reports]
from CrewMembers Flash
join (
    select Barry.crewMemberId,
        AVG(Zoom.salary) as 'AVGSalary',
        COUNT(Zoom.firstname) as 'Direct Reports'
    from CrewMembers Barry
    join CrewMembers Zoom on Zoom.managerId = Barry.crewMemberId
    group by Barry.crewMemberId
    having avg(Zoom.salary) = (
            select min(salary)
            from (
                select AVG(Zoom.salary) as salary
                from CrewMembers Barry
                join CrewMembers Zoom on Zoom.managerId = Barry.crewMemberId
                group by Barry.crewMemberId
                ) t
            )
    ) as Wally on Wally.crewMemberId = Flash.crewMemberId

你可以使用top:

select top 1 Flash.firstname,
    Flash.lastname,
    Wally.AVGSalary,
    Wally.[Direct Reports]
from CrewMembers Flash
join (
    select Barry.crewMemberId,
        AVG(Zoom.salary) as 'AVGSalary',
        COUNT(Zoom.firstname) as 'Direct Reports'
    from CrewMembers Barry
    join CrewMembers Zoom on Zoom.managerId = Barry.crewMemberId
    group by Barry.crewMemberId
    ) as Wally on Wally.crewMemberId = Flash.crewMemberId
order by Wally.AVGSalary

另一种方法是使用窗口函数rank：

select Flash.firstname,
    Flash.lastname,
    Wally.AVGSalary,
    Wally.[Direct Reports]
from CrewMembers Flash
join (
    select Barry.crewMemberId,
        AVG(Zoom.salary) as 'AVGSalary',
        COUNT(Zoom.firstname) as 'Direct Reports',
        rank() over (order by AVG(Zoom.salary)) as rnk
    from CrewMembers Barry
    join CrewMembers Zoom on Zoom.managerId = Barry.crewMemberId
    group by Barry.crewMemberId
    ) as Wally on Wally.crewMemberId = Flash.crewMemberId
where Wally.rnk = 1;

Answer 2

你应该使用 HAVING 和 equals 来代替 IN 所以类似：

HAVING Wally.AVGSalary = MIN(Wally.AVGSalary)

在您使用IN 的情况下，它将返回所有行，并且您只想要薪水等于最低的行。

Answer 3

试试这个：

Select crewMemberId, avgSalary
From (Select s.crewMemberId, 
         AVG(d.salary) avgSalary
      From CrewMembers d join CrewMembers s 
         on s.crewMemberId = d.managerId 
      group by s.crewMemberId) x
Where avgSalary = 
   (Select Min(avgSalary) 
    from (Select s.crewMemberId, 
             AVG(d.salary) avgSalary
          From CrewMembers d join CrewMembers s 
              on s.crewMemberId = d.managerId 
          group by s.crewMemberId)x)       

-- 在临时表变量中使用样本数据：

declare @e table(crewmemberId int primary key not null, 
              managerId int null, salary decimal)
Insert @e(crewmemberId, managerId, salary)
values (1, null, 23), 
       (2,1,45), (3,1,33), (4,1,80), 
       (5,2,14), (6,2,8), (7,2,12),
       (8,3,11), (9,3,5), (10,3,2), 
       (11,4,51), (12,4,38), (13,4,17) 

Select crewMemberId, avgSalary
From (Select s.crewMemberId, 
         AVG(d.salary) avgSalary
      From @e d join @e s 
         on s.crewMemberId = d.managerId 
      group by s.crewMemberId) x
Where avgSalary = 
   (Select Min(avgSalary)
    From (Select s.crewMemberId, 
            AVG(d.salary) avgSalary
          From @e d join @e s 
             on s.crewMemberId = d.managerId 
          group by s.crewMemberId)x)        

这给出了crewmemberId = 3, Avg Salary = 6.0000;

如果直接聘用两个或多个经理的平均工资相同，则具有产生多个结果的优势。