Question

我有三个表TableThreeItems，TableTwoItems和TableOneItems，我需要获取Person在每个表中的访问次数。

我尝试了以下内容：

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]

但是这并不认为每个人的计数是分开的，但我总是错误的计数。例如，如果他有1个TableOneItem和2个TableTwoItemIds，则TableOneItemCount为2，而TableTwoItemIdCount为2

Answer 1

只需在计数中添加DISTINCT即可：

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 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

您可以使用join：

在一个count(distinct)中执行此操作

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];

如果您希望计数甚至适度大，则不建议这样做。如果计数均为100，那么这将为每个人生成100 * 100 * 100 = 1,000,000个中间行的笛卡尔积。处理方式太多了。在这种情况下，您希望将计数作为子查询进行。

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;

在大多数情况下，您只需在子查询中使用group by PersonId进行计算，而不使用where。但你只选择了一个人。

Answer 4

join将为每个组合创建一个行，并且仅在之后应用分组。因此，正如您所指出的，这不会生成正确的输出。一个技巧是在加入之前执行分组和计算：

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

从多个表中选择计数

4 个答案: