连接两个表，仅使用右表的最新值 [英] Join two tables, only use latest value of right table

问题描述

我正在尝试联接2个表，但仅与一组记录中的最新记录联接.

I am trying to join 2 tables, but only join with the latest record in a group of records.

左表:

零件

• Part.PartNum

右表:

材料

• Material.Partnum
• Material.Formula
• Material.RevisionNum

修订号从"A"开始并增加.

The revision number starts at "A" and increases.

我想加入PartNum的2个表，但是只合并右边表中的最新记录.我已经看到了有关SO的其他示例，但是很难将它们放在一起.

I would like to join the 2 tables by PartNum, but only join with the latest record from right table. I have seen other examples on SO but an having a hard time putting it all together.

我发现第一个修订版本号是"New"，然后它递增A，B，...它永远不会超过一个或两个修订版本，因此我不必担心遍历该序列.但是，如何选择以"New"作为第一个修订版本号的最新版本?

I found out the first revision number is "New", then it increments A,B,... It will never be more than one or two revisions, so I am not worried about going over the sequence. But how do I choose the latest one with 'New' being the first revision number?

推荐答案

将运行此常规SQL语句为:

A general SQL statement that would run this would be:

select P.PartNum, M.Formula, M.RevisionNum
from Parts P
join Material M on P.PartNum = M.PartNum
where M.RevisionNum = (select max(M2.RevisionNum) from Material M2
                       where M2.PartNum = P.PartNum);

重复以上有关第26版修订之后的注意事项. max(RevisionNum)可能会中断，具体取决于#26之后发生的情况.

Repeating the above caveats about what happens after Revision #26. The max(RevisionNum) may break depending upon what happens after #26.

如果RevisionNum序列始终从w/NEW开始，然后以A，B，C等继续，则需要替换w()并添加一些更复杂(且混乱)的东西:

If RevisionNum sequence always starts w/ NEW and then continues, A, B, C, etc., then the max() needs to be replaced w/ something more complicated (and messy):

select P.PartNum, M.RevisionNum
from Parts P
join Material M on P.PartNum = M.PartNum
where (
      (select count(*) from Material M2 
              where M2.PartNum = P.PartNum) > 1
      and M.RevisionNum = (select max(M3.RevisionNum) from Material M3
                   where M3.PartNum = P.PartNum and M3.RevisionNum <> 'NEW')
      )
      or ( 
      (select count(*) from Material M4
              where M4.PartNum = P.PartNum) = 1
       and M.RevisionNum = 'NEW'
      )

必须有一种更好的方法来执行此操作.尽管这行得通-必须考虑一个更快的解决方案.

There must be a better way to do this. This works though -- will have to think about a faster solution.

SQL Fiddle: http://sqlfiddle.com/#!3/70c19/3

SQL Fiddle: http://sqlfiddle.com/#!3/70c19/3

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