比较触发器中的字段

Question

所以我创建了一个触发器，用于比较更新前后的内容，并确定 where 子句中指定的特定字段是否已更改。如果是这样，我将先前信息的快照插入到历史表中。

问题是当字段创建时使用空值（根据业务规则，它是合法的空值）并且在进行更新时，它无法评估字符串不等于空值。我想捕捉到它是空的，后来又被填充了。

我应该使用什么方法来比较空字段而不影响性能？

这适用于 SQL Server 2005

CREATE TRIGGER [prj].[TRG_Master_Projection_Upd_History] ON [prj].[Master_Projections]
AFTER UPDATE
AS
    SET NOCOUNT ON -- Prevents the error that gets thrown after inserting multiple records 
                   -- if multiple records are being updated

BEGIN


INSERT INTO prj.History_Projections (ProjectionID, Cancelled, Appeal, Description, Response_PatternID, 
                                     Proj_Mail_date, [3602_Mail_Date], Proj_Qty, [3602_Qty], Proj_Resp_Rate,
                                     Bounce_Back, Nickels, Kits, Oversized_RE, ChangeComments, 
                                     Modification_Process, Modification_Date, Modification_User)
    SELECT D.ProjectionID, D.Cancelled, D.Appeal, D.Description, D.Response_PatternID, D.Proj_Mail_Date,
            D.[3602_Mail_Date], D.Proj_Qty, D.[3602_Qty], D.Proj_Resp_Rate, D.Bounce_Back, D.Nickels, D.Kits,
            D.Oversized_RE, D.ChangeComments, D.Modification_Process, D.Modification_Date, D.Modification_User
       FROM deleted as D
    JOIN inserted as I
         ON D.ProjectionID = I.ProjectionID
      WHERE (I.Cancelled <> D.Cancelled
          OR I.Appeal <> D.Appeal
          OR I.Description <> D.Description
          OR I.Response_PatternID <> D.Response_PatternID
          OR I.Proj_Mail_Date <> D.Proj_Mail_Date
          OR I.[3602_Mail_Date] <> D.[3602_Mail_Date]
          OR I.Proj_Qty <> D.Proj_Qty
          OR I.[3602_Qty] <> D.[3602_Qty]
          OR I.Proj_Resp_Rate <> D.Proj_Resp_Rate
          OR I.Bounce_Back <> D.Bounce_Back
          OR I.Nickels <> D.Nickels
          OR I.Kits <> D.Kits
          OR I.Oversized_RE <> D.Oversized_RE )
END;
SET NOCOUNT OFF;

Answer 1

不幸的是，你真的必须使用哨兵值

ISNULL(I.Response_PatternID, 0) <> ISNULL(D.Response_PatternID, 0)

不幸的是，没有魔法：您必须比较每个值才能看到任何差异。

也就是说，您只触摸 INSERTED 和 DELETED 表，如此糟糕，主表没有被触及。除非您有影响 10000 行的更新，否则它将运行正常。

你也可以使用 OR ，但这很麻烦

(I.Response_PatternID <> D.Response_PatternID OR I.Response_PatternID IS NULL AND I.Response_PatternID IS NOT NULL OR I.Response_PatternID IS NOT NULL AND I.Response_PatternID IS NULL) 

我会坚持使用 ISNULL 以避免 COALESCE 出现微妙的数据类型问题