從多個標籤中選擇計數

Question

我有三個表格TableThreeItems，TableTwoItems和TableOneItems，我需要獲取Person在這些表格中每個表格中的訪問次數。

我試過如下：

SELECT Person.[PersonId] 

, Count(TableOneItemId) as TableOneItemCount 
, Count(TableTwoItemIdId) as TableTwoItemCount 
, Count(TableThreeItemId) as TableThreeItemCount 


FROM [dbo].[Person] 
LEFT JOIN TableOneItem ON TableOneItem.PersonId=Person.PersonId 
LEFT JOIN TableThreeItemId ON TableThreeItemId.PersonId=Person.PersonId 
LEFT JOIN TableTwoItemId ON TableTwoItemId.PersonId=Person.PersonId 
WHERE [Person].PersonId=1 
GROUP BY Person.[PersonId] 

但這並不考慮每一種作爲單獨的計數，但我總是一個錯誤的數。舉例來說，如果他有1 TableOneItem和2個TableTwoItemIds的TableOneItemCount將是2，而TableTwoItemIdCount是2

Answer 1

您可以通過使用count(distinct)單一join做到這一點：

SELECT Person.[PersonId], 
     Count(distinct TableOneItemId) as TableOneItemCount, 
     Count(distinct TableTwoItemIdId) as TableTwoItemCount, 
     Count(distinct TableThreeItemId) as TableThreeItemCount 
FROM [dbo].[Person] LEFT JOIN 
     TableOneItem 
     ON TableOneItem.PersonId = Person.PersonId LEFT JOIN 
     TableThreeItemId 
     ON TableThreeItemId.PersonId = Person.PersonId LEFT JOIN 
     TableTwoItemId 
     ON TableTwoItemId.PersonId = Person.PersonId 
WHERE [Person].PersonId = 1 
GROUP BY Person.[PersonId]; 

這是如果您希望計數甚至適中，則不推薦使用。如果計數爲100，那麼這將爲每個具有100 * 100 * 100 = 1,000,000中間行的人產生笛卡爾積。處理方式太多。在這種情況下，您希望將計數作爲子查詢。

SELECT Person.[PersonId], TableOneItemCount, TableTwoItemCount, TableThreeItemCount 
FROM [dbo].[Person] LEFT JOIN 
     (select personid, count(TableOneItemId) as TableOneItemCount from TableOneItem where PersonId = 1 group by personid 
    ) t1 
     ON t1.PersonId = Person.PersonId LEFT JOIN 
     (select personid, count(TableTwoItemId) as TableTwoItemCount from TableTwoItem where PersonId = 1 group by personid 
    ) t2 
     ON t2.PersonId = Person.PersonId LEFT JOIN 
     (select personid, count(TableThreeItemId) as TableThreeItemCount from TableThreeItem where PersonId = 1 group by personid 
    ) t3 
     ON t3.PersonId = Person.PersonId 
WHERE Person.PersonId = 1; 

在大多數情況下，你只需使用group by PersonId中的子查詢，而不where計算。但你只選擇一個人。

Answer 2

SELECT Person.[PersonId] 

, (SELECT Count(TableOneItemId) from TableOneItem where TableOneItem.PersonId=Person.PersonId) as TableOneItemCount 
, (SELECT Count(TableTwoItemId) from TableTwoItem where TableTwoItem.PersonId=Person.PersonId) as TableTwoItemCount 
, (SELECT Count(TableThreeItemId) from TableThreeItem where TableThreeItem.PersonId=Person.PersonId) as TableThreeItemCount 

FROM [dbo].[Person] 
WHERE [Person].PersonId=1 

Answer 3

只需添加一個DISTINCT來計數，就大功告成了：

SELECT Person.[PersonId] 

, Count(DISTINCT TableOneItemId) as TableOneItemCount 
, Count(DISTINCT TableTwoItemIdId) as TableTwoItemCount 
, Count(DISTINCT TableThreeItemId) as TableThreeItemCount 


FROM [dbo].[Person] 
LEFT JOIN TableOneItem ON TableOneItem.PersonId=Person.PersonId 
LEFT JOIN TableThreeItemId ON TableThreeItemId.PersonId=Person.PersonId 
LEFT JOIN TableTwoItemId ON TableTwoItemId.PersonId=Person.PersonId 
WHERE [Person].PersonId=1 
GROUP BY Person.[PersonId] 

Answer 4

join ING會爲每種組合創建一個行，分組將只應用之後。所以，如你所說，這不會產生正確的輸出。一個技巧就是你加入之前進行分組和計數：

SELECT Person.[PersonId], TableOneItemCount, TableTwoItemCount, TableThreeItemCount 
FROM  Person 
LEFT JOIN (SELECT PersonId, COUNT(*) AS TableOneItemCount 
      FROM  TableOneItem 
      GROUP BY PersonId) t1 ON p.PersonId = t1.PersonId 
LEFT JOIN (SELECT PersonId, COUNT(*) AS TableTwoItemCount 
      FROM  TableTwoItem 
      GROUP BY PersonId) t2 ON p.PersonId = t2.PersonId 
LEFT JOIN (SELECT PersonId, COUNT(*) AS TableThreeItemCount 
      FROM  TableThreeItem 
      GROUP BY PersonId) t3 ON p.PersonId = t3.PersonId 
WHERE  [Person].PersonId = 1