问题描述
我的查询要求我按人数(附加示意图)找到最繁忙的位置
select disTINCT location.name as 'Location',f_name|| ' ' || l_name as 'Citizen'
from CheckIn
join location on checkin.LociD = Location.LociD
join person on person.PersonID = CheckIn.PersonID
通过以上查询。我可以找到拜访过该地点的人,但是我找不到的拜访该地点的人最多,因为他们都是单独显示的。我知道我需要添加一个计数或分组依据。
如果我尝试按以下方式计数
select disTINCT location.name as 'Location',f_name|| ' ' || l_name as 'Citizen',count (personid)
from CheckIn
join location on checkin.LociD = Location.LociD
join person on person.PersonID = CheckIn.PersonID
它将显示出模糊的列。我知道这是因为我已经将它用作联接,但是如果我不能重用它,那么我该如何计算人数?
解决方法
此查询:
SELECT LID,COUNT(*)
FROM CheckIn
GROUP BY LID
返回每个LID
访问的人数。
您还可以使用窗口功能RANK()
按人数对每个位置进行排名:
SELECT LID,RANK() OVER (ORDER BY COUNT(*) DESC) rnk
FROM CheckIn
GROUP BY LID
最后,您将此查询与运算符IN
一起使用,以返回排名第一的位置的详细信息:
SELECT * FROM Location
WHERE LocationID IN (
SELECT LID
FROM (
SELECT LID,RANK() OVER (ORDER BY COUNT(*) DESC) rnk
FROM CheckIn
GROUP BY LID
)
WHERE rnk = 1
)
,
您可以尝试以下操作:
select
name,COUNT(P.PersonID) as People_Count
from Location AS L
inner join checkin as C on C.LocID = L.LocID
inner join person as P on P.PersonID = C.PersonID
group by name
order by COUNT(P.PersonID) desc