MySQL通過不返回正確的結果進行分組

Question

我有一個正在嘗試生成報告的票務系統。 我正在嘗試獲取每個用戶涉及的票數。 有了第一個查詢：

SELECT * FROM (
SELECT TicketID, UserID, EventDateTime
FROM dcscontact.ticketevents
WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
ORDER BY EventDateTime DESC) x
WHERE UserID=80
GROUP BY TicketID;

我能夠列出針對特定用戶的票證，並且可以手動對其進行計數：

TicketID    UserID  EventDateTime
99168       80      6/22/2016 13:21
99193       80      6/22/2016 7:42
99213       80      6/22/2016 13:02
99214       80      6/22/2016 6:30
99221       80      6/22/2016 6:57
99224       80      6/22/2016 7:48
99226       80      6/22/2016 6:27
99228       80      6/22/2016 8:49
99229       80      6/22/2016 8:53
99232       80      6/22/2016 9:18
99237       80      6/22/2016 13:08

但是，當我嘗試刪除WHERE UserID =語句，並嘗試將其用作子查詢時，如下所示：

SELECT UserID, COUNT(*) as count FROM (
    SELECT * FROM (
    SELECT TicketID, UserID, EventDateTime
    FROM dcscontact.ticketevents
    WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
    ORDER BY EventDateTime DESC) x
    GROUP BY TicketID) y
GROUP BY UserID;

我得到不正確的計數：

UserID  count
9       2
28      1
31      1
42      1
80      5
95      1
99      6
108     4
116     12
117     26
123     24

如您所見，UserID 80的計數應該為11。其他大多數結果也不正確，它們似乎都比我預期的要少。

在子查詢上使用GROUP BY / COUNT時，我做錯什么了嗎？ 如何更改查詢以獲得所需的結果？

Answer 1

您是否只想匯總？

SELECT UserID, COUNT(*)
FROM dcscontact.ticketevents
WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
GROUP BY UserID;

如果對於一個給定用戶，同一票證可以多次出現在數據中，則COUNT(DISTINCT)更合適：

SELECT UserID, COUNT(DISTINCT TicketID)
FROM dcscontact.ticketevents
WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
GROUP BY UserID;

Answer 2

要獲取每個用戶觸摸的票證數量 ，讓我們從正確的查詢開始：

SELECT count(*) as N, UserID
FROM dcscontact.ticketevents
WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
GROUP BY UserID;

GROUP BY子句應始終包括SELECT子句中提到的所有非聚合列。 詢問“票證ID和票證數量（每位用戶）”是沒有意義的！

另外，SQL標准說ORDER BY不能應用於子查詢。 最好將ORDER BY視為查看輸出的便利，而不是查詢中使用的信息。

您還想知道一些關於TicketID和EventDateTime 。 您不能要求輸入“票證編號”，但可以獲取第一張和最后一張票證。 時間相同：

SELECT   count(*) as N
       , min(TicketID) as T1
       , max(TicketID) as Tn
       , min(EventDateTime) as E1
       , max(EventDateTime) as En
   , UserID
FROM dcscontact.ticketevents
WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
GROUP BY UserID;

請注意，最早的時間可能不是最小的TicketID的時間。 要獲取有關每個用戶的第一張票證以及計數的所有信息，請結合以下兩種信息來源：

select N.N, T.* 
from dcscontact.ticketevents as T
join (
    SELECT count(*) as N, min(TicketID) as T1, UserID
    FROM dcscontact.ticketevents
    WHERE EventDateTime BETWEEN '2016-06-22' AND '2016-06-23'
    GROUP BY UserID;
) as N
on  T.UserID = N.UserID 
and T.TicketID = N.TicketID
-- and maybe others, according to the key
order by EventDateTime DESC