学习 SQL：需要帮助使用 HAVING 从聚合中获取 MIN 结果

Question

希望得到一些帮助。 SQL 的新手，努力保持头脑清醒。

我接到了这个任务：

"Using HAVING, determine whose Direct Reports have the lowest Avg Salary."

我的逻辑是对派生的 table 进行联接，该派生的 table 将计算每个有直接下属的经理的工作人员平均工资。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 JUST 具有最小值的行，它给了我每个值。我盯着这个看，不知道我做错了什么。

我知道，通过在线查看线程，使用 TOP 是一种过滤项目的方法，但尚未介绍，我想保持在课程中已建立的参数范围内，所以远的。

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

你需要另一个子查询（如果你不想使用 top/window function/etc）：

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

您可以使用 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 window 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

您应该将 HAVING 与等于而不是 IN 一起使用，例如：

HAVING Wally.AVGSalary = MIN(Wally.AVGSalary)

在你 IN 的情况下，它将 return 所有行，你只想要薪水等于最低的人。

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 table 变量中使用示例数据：

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;

如果直接雇用两个或更多经理的平均工资相同，它的优势是产生多个结果。

学习 SQL：需要帮助使用 HAVING 从聚合中获取 MIN 结果

Learning SQL: Need help Using HAVING to get the MIN result from an aggregate

sql

sql-server

subquery

min

having