Question

我有一个不是由我创建的存储过程。

在某些存储过程中，存在嵌套连接，我很难理解。

如何更简单地编写它以理解逻辑？还有那些被使用的原因？仅提高绩效？

SELECT *
FROM      
    dbo.tblQuotes AS tblQuotes_1 
LEFT OUTER JOIN
    dbo.lstLines ON tblQuotes_1.LineGUID = dbo.lstLines.LineGUID 
RIGHT OUTER JOIN
    dbo.tblUsers 
RIGHT OUTER JOIN
    dbo.tblQuotes 
RIGHT OUTER JOIN
    dbo.tblClaims_Claim ON dbo.tblQuotes.ControlNo = dbo.tblClaims_Claim.ControlNo 
RIGHT OUTER JOIN
    dbo.tblNoteEntries 
LEFT OUTER JOIN
    dbo.tblNoteRecipients ON dbo.tblNoteEntries.ID = dbo.tblNoteRecipients.EntryGUID 
INNER JOIN
    dbo.tblNoteDiaries ON dbo.tblNoteEntries.ID = dbo.tblNoteDiaries.EntryGUID 
INNER JOIN
    dbo.tblNoteEntities ON dbo.tblNoteEntries.NoteGUID = dbo.tblNoteEntities.NoteGUID 
INNER JOIN
    dbo.tblNoteStore ON dbo.tblNoteEntries.NoteGUID = dbo.tblNoteStore.ID 
                     AND dbo.tblNoteDiaries.NoteGUID = dbo.tblNoteStore.ID 
                     AND dbo.tblNoteEntities.NoteGUID = dbo.tblNoteStore.ID 
INNER JOIN
    dbo.lstNoteTypes ON dbo.tblNoteStore.Type = dbo.lstNoteTypes.NoteTypeID 
       ON dbo.tblClaims_Claim.ClaimGuid = dbo.tblNoteEntities.AssociatedEntityGUID 
       ON dbo.tblUsers.UserGUID = dbo.tblNoteRecipients.UserGUID 
       ON tblQuotes_1.ControlGuid = dbo.tblNoteEntities.ControlGuid 
LEFT OUTER JOIN
    dbo.tblMaxQuoteIDs ON tblQuotes_1.QuoteID = dbo.tblMaxQuoteIDs.MaxQuoteID 
LEFT OUTER JOIN
    dbo.tblInsureds ON dbo.tblNoteEntities.AssociatedEntityGUID = dbo.tblInsureds.InsuredGUID

Answer 1

我通常将列表转换为缩进列表，就好像各种连接是嵌套的IF语句一样。例如：

SELECT *
FROM dbo.tblQuotes AS tblQuotes_1 
LEFT OUTER JOIN dbo.lstLines 
    ON tblQuotes_1.LineGUID = dbo.lstLines.LineGUID 
RIGHT OUTER JOIN dbo.tblUsers 
    RIGHT OUTER JOIN dbo.tblQuotes 
        RIGHT OUTER JOIN dbo.tblClaims_Claim 
            ON dbo.tblQuotes.ControlNo = dbo.tblClaims_Claim.ControlNo 
        RIGHT OUTER JOIN dbo.tblNoteEntries 
            LEFT OUTER JOIN dbo.tblNoteRecipients 
                ON dbo.tblNoteEntries.ID = dbo.tblNoteRecipients.EntryGUID 
            INNER JOIN dbo.tblNoteDiaries 
                ON dbo.tblNoteEntries.ID = dbo.tblNoteDiaries.EntryGUID 
            INNER JOIN dbo.tblNoteEntities 
                ON dbo.tblNoteEntries.NoteGUID = dbo.tblNoteEntities.NoteGUID 
            INNER JOIN dbo.tblNoteStore 
                ON dbo.tblNoteEntries.NoteGUID = dbo.tblNoteStore.ID 
                AND dbo.tblNoteDiaries.NoteGUID = dbo.tblNoteStore.ID 
                AND dbo.tblNoteEntities.NoteGUID = dbo.tblNoteStore.ID 
            INNER JOIN dbo.lstNoteTypes 
                ON dbo.tblNoteStore.Type = dbo.lstNoteTypes.NoteTypeID 
            ON dbo.tblClaims_Claim.ClaimGuid = dbo.tblNoteEntities.AssociatedEntityGUID 
        ON  dbo.tblUsers.UserGUID = dbo.tblNoteRecipients.UserGUID 
    ON tblQuotes_1.ControlGuid = dbo.tblNoteEntities.ControlGuid 
LEFT OUTER JOIN dbo.tblMaxQuoteIDs 
    ON tblQuotes_1.QuoteID = dbo.tblMaxQuoteIDs.MaxQuoteID 
LEFT OUTER JOIN dbo.tblInsureds 
    ON dbo.tblNoteEntities.AssociatedEntityGUID = dbo.tblInsureds.InsuredGUID

然后我将任何不合适的ON条件移动到正确的位置，如果我可以在不破坏查询逻辑的情况下执行它。在这种情况下，如果没有一些重要的分析，我就无法做到。

注意：老实说，我讨厌这种类型的连接序列，如果你知道表和关系，你可以将它重新组织成一个简单的连接列表，而不是奇怪的嵌套连接结构。但这取决于两件重要的事情：

这样做是为了优化查询的性能吗？

即使是现在，在我使用的最新版本的SQL Server（2014）中，也有一些查询可以通过在JOIN列表中移动表引用来优化。如果这就是将列表重组为此嵌套结构的原因，则可能会通过撤消特殊组织严重损害性能。当然，如果这是在最初的SQL Server版本中编写的，您可能需要解开这些联接，以查看查询优化器现在是否能很好地处理此结构。

这样做是因为数据规范化存在一些缺陷，实际上需要嵌套的JOIN列表才能工作吗？

永远不要低估前任的愚蠢（例如，访问The Daily WTF并浏览几分钟。）如果数据结构可以以需要笨拙和非规范化JOIN的方式相关联，将以这种方式相关。我还没有真正看到这个原因得到验证，但我知道在某些地方有一些数据结构需要这个。

Answer 2

如果可读性是您的目标，那么值得注意的是您根本不需要嵌套或RIGHT JOIN。以下

在数学上等效
从左到右一直读取（假设您喜欢这种方式）
没有嵌套
没有合适的人选

少了不必要的字符和行

SELECT *
  FROM dbo.tblNoteStore              nt_str
           JOIN dbo.lstNoteTypes     nt_typ ON nt_str.[Type] = nt_typ.NoteTypeID
           JOIN dbo.tblNoteDiaries   nt_dia ON nt_str.ID = nt_dia.NoteGUID
           JOIN dbo.tblNoteEntities   entit ON nt_str.ID = entit.NoteGUID
           JOIN dbo.tblNoteEntries    entri ON nt_str.ID = entri.NoteGUID
                                               and nt_dia.EntryGUID = entri.ID 
                                               and entit.NoteGUID = entri.NoteGUID
      LEFT JOIN dbo.tblNoteRecipients recip ON entri.ID = recip.EntryGUID
      LEFT JOIN dbo.tblUsers            usr ON recip.UserGUID = usr.UserGUID
      LEFT JOIN dbo.tblInsureds       insur ON entit.AssociatedEntityGUID = insur.InsuredGUID
      LEFT JOIN dbo.tblClaims_Claim   claim ON entit.AssociatedEntityGUID = claim.ClaimGUID
      LEFT JOIN dbo.tblQuotes         qts_1 ON entit.ControlGUID = qts_1.ControlGUID
      LEFT JOIN dbo.lstLines          lines ON qts_1.LineGUID = lines.LineGUID
      LEFT JOIN dbo.tblMaxQuoteIDs   max_qt ON qts_1.QuoteID = max_qt.MaxQuoteId
      LEFT JOIN dbo.tblQuotes         qts_2 ON claim.ControlNo = qts_2.ControlNo 
      ;

如何在SQL Server 2012中分解嵌套的JOIN以便正确读取它

2 个答案: