左外连接中的最大日期

Question

感谢上一个问题，我发现如何根据链接表提取最新数据。但是，现在我有一个相关的问题。

我找到的解决方案使用了row_number（）和PARTITION来提取最近一组数据。但是如果视图中的链接表中有零个或多个行的可能性呢？例如，表FollowUpDate可能有0行，或1或更多。我只是想最近FollowUpDate：

SELECT 
    EFD.FormId 
    ,EFD.StatusName 
    ,MAX(EFD.ActionDate) 
    ,EFT.Name AS FormType 
    ,ECOA.Account AS ChargeOffAccount 
    ,ERC.Name AS ReasonCode 
    ,EAC.Description AS ApprovalCode 
    ,MAX(EFU.FollowUpDate) AS FollowUpDate 
FROM (
    SELECT EF.FormId, EFD.ActionDate, EFS.Name AS StatusName, EF.FormTypeId, EF.ChargeOffId, EF.ReasonCodeId, EF.ApprovalCodeId, 
     row_number() OVER (PARTITION BY EF.FormId ORDER BY EFD.ActionDate DESC) DateSortKey 
    FROM Extension.FormDate EFD INNER JOIN Extension.Form EF ON EFD.FormId = EF.FormId INNER JOIN Extension.FormStatus EFS ON EFD.StatusId = EFS.StatusId 
) EFD 
    INNER JOIN Extension.FormType EFT ON EFD.FormTypeId = EFT.FormTypeId 
    LEFT OUTER JOIN Extension.ChargeOffAccount ECOA ON EFD.ChargeOffId = ECOA.ChargeOffId 
    LEFT OUTER JOIN Extension.ReasonCode ERC ON EFD.ReasonCodeId = ERC.ReasonCodeId 
    LEFT OUTER JOIN Extension.ApprovalCode EAC ON EFD.ApprovalCodeId = EAC.ApprovalCodeId 
    LEFT OUTER JOIN (Select EFU.FormId, EFU.FollowUpDate, row_number() OVER (PARTITION BY EFU.FormId ORDER BY EFU.FollowUpDate DESC) FUDateSortKey FROM Extension.FormFollowUp EFU INNER JOIN Extension.Form EF ON EFU.FormId = EF.FormId) EFU ON EFD.FormId = EFU.FormId 
WHERE EFD.DateSortKey = 1 
GROUP BY 
    EFD.FormId, EFD.ActionDate, EFD.StatusName, EFT.Name, ECOA.Account, ERC.Name, EAC.Description, EFU.FollowUpDate 
ORDER BY 
    EFD.FormId 

如果我做了类似的使用上拉ROW_NUMBER（）和分区，我得到的数据只是如果在FollowUpDate至少一行。有点击败了左外连接的目的。任何人都可以帮助我做到这一点？

Answer 1

我改写了您的查询 - 你有不必要的子查询，并用ROW_NUMBER（）为FUDat​​eSortKey但没有使用的列：

SELECT t.formid, 
    t.statusname, 
    MAX(t.actiondate) 'actiondate', 
    t.formtype, 
    t.chargeoffaccount, 
    t.reasoncode, 
    t.approvalcode, 
    MAX(t.followupdate) 'followupdate' 
FROM (
    SELECT t.formid, 
      fs.name 'StatusName', 
      t.actiondate, 
      ft.name 'formtype', 
      coa.account 'ChargeOffAccount', 
      rc.name 'ReasonCode', 
      ac.description 'ApprovalCode', 
      ffu.followupdate, 
      row_number() OVER (PARTITION BY ef.formid ORDER BY t.actiondate DESC) 'DateSortKey' 
    FROM EXTENSION.FORMDATE t 
    JOIN EXTENSION.FORM ef ON ef.formid = t.formid 
    JOIN EXTENSION.FORMSTATUS fs ON fs.statusid = t.statusid 
    JOIN EXTENSION.FORMTYPE ft ON ft.formtypeid = ef.formtypeid 
LEFT JOIN EXTENSION.CHARGEOFFACCOUNT coa ON coa.chargeoffid = ef.chargeoffid 
LEFT JOIN EXTENSION.REASONCODE rc ON rc.reasoncodeid = ef.reasoncodeid 
LEFT JOIN EXTENSION.APPROVALCODE ac ON ac.approvalcodeid = ef.approvalcodeid 
LEFT JOIN EXTENSION.FORMFOLLOWUP ffu ON ffu.formid = t.formid) t 
    WHERE t.datesortkey = 1 
GROUP BY t.formid, t.statusname, t.formtype, t.chargeoffaccount, t.reasoncode, t.approvalcode 
ORDER BY t.formid 

改变我允许FollowUpDate是在FORMFOLLOWUP表上使用一个LEFT JOIN--你正在做一个INNER JOIN，所以你只能得到FORMFOLLOWUP记录关联的行。

Answer 2

如果没有表格定义和样本数据，很难猜测发生了什么。

此外，这是令人困惑的：“表FollowUpDate可能有0行”，你“希望最新的FollowUpDate”。 （特别是当没有名为FollowUpDate的表时）如果没有FollowUpDate，则没有“最近的FollowUpDate”。

也许你想

WHERE <follow up date row number> in (1,NULL) 

Answer 3

我想通了。和往常一样，我需要小睡一下。我只是需要改变我的选择，我会发誓我试过没有成功：

SELECT field1, field2  
FROM Table1 t1  
LEFT JOIN ( 
     SELECT field3, max(dateColumn)  
     FROM Table2  
     GROUP BY  
      field3  
) t2  
ON (t1.field1 = t2.field3)