在 WHERE 中使用 CASE 表达式进行 JOIN 优化

Question

下面的 2 个表将从 details_2 表中提取大量数据，但会根据 #exceptions 表中编码的不同原因加入字段。

有没有更好的方法来编写以下查询？

Select
a.Fieldname1,
a.Fieldname2,
a.fieldname3
from 
#Exceptions a, [DETAILS_2] b
where 

(case when a.REASON_CD = 'LOBMKT' then CONCAT(a.LOB_CD,a.GEO_MARKET) end = concat(b.LOB_CD,b.GEO_MARKET)) or
(case  when a.REASON_CD = 'LOB' then a.LOB_CD end  = b.LOB_CD) or
(case  when a.REASON_CD = 'CAT' then a.CTRCT_CAT_CD end  = b.CTRCT_CAT_CD) or
(case  when a.REASON_CD = 'LOBST' then CONCAT(a.LOB_CD,a.[derived_GEO_MARKET]) end = concat(b.LOB_CD,b.GEO_MARKET)) 

Answer 1

我想我会为此使用 go，但是如果没有样本数据就无法测试任何东西，并且需要 output，就像 Cetin 也提到的那样。

对于 UNION ALL，我会使用 go，因为数据在所有不同的语句中应该是唯一的。 所以 SQL 不需要验证（消耗更多 CPU）

SELECT
     a.Fieldname1,
     a.Fieldname2,
     a.fieldname3
FROM #Exceptions a
     JOIN [DETAILS_2] b ON a.REASON_CD = 'LOBMKT'
                           AND a.LOB_CD = b.LOB_CD
                           AND a.GEO_MARKET = b.GEO_MARKET
UNION ALL
SELECT
     a.Fieldname1,
     a.Fieldname2,
     a.fieldname3
FROM #Exceptions a
     JOIN [DETAILS_2] b ON a.REASON_CD = 'LOB'
                           AND a.LOB_CD = b.LOB_CD
UNION ALL
SELECT
     a.Fieldname1,
     a.Fieldname2,
     a.fieldname3
FROM #Exceptions a
     JOIN [DETAILS_2] b ON a.REASON_CD = 'CAT'
                           AND a.CTRCT_CAT_CD = b.CTRCT_CAT_CD
UNION ALL
SELECT
     a.Fieldname1,
     a.Fieldname2,
     a.fieldname3
FROM #Exceptions a
     JOIN [DETAILS_2] b ON a.REASON_CD = 'LOBST'
                           AND a.LOB_CD = b.LOB_CD
                           AND a.[derived_GEO_MARKET] = b.GEO_MARKET;

Answer 2

这种方法是可搜索的并且更容易理解它在做什么：

Select a.Fieldname1, a.Fieldname2, a.fieldname3
from #Exceptions a
inner join [DETAILS_2] b on a.LOB_CD = b.LOB_CD and a.GEO_MARKET = b.GEO_MARKET
where a.REASON_CD = 'LOBMKT'
UNION ALL
Select a.Fieldname1, a.Fieldname2, a.fieldname3
from #Exceptions a
inner join [DETAILS_2] b on a.LOB_CD = b.LOB_CD
where a.REASON_CD = 'LOB'
UNION ALL
Select a.Fieldname1, a.Fieldname2, a.fieldname3
from #Exceptions a
inner join [DETAILS_2] b on a.CTRCT_CAT_CD = b.CTRCT_CAT_CD
where a.REASON_CD = 'CAT'
UNION ALL
Select a.Fieldname1, a.Fieldname2, a.fieldname3
from #Exceptions a
inner join [DETAILS_2] b on a.LOB_CD = b.LOB_CD and a.[derived_GEO_MARKET] = b.GEO_MARKET
where a.REASON_CD = 'LOBST'

Answer 3

（当心你正在做交叉连接）

SELECT a.Fieldname1,
       a.Fieldname2,
       a.fieldname3
FROM #Exceptions a,
     [DETAILS_2] b
WHERE (
          a.REASON_CD = 'LOBMKT'
          AND a.LOB_CD = b.LOB_CD
          AND a.GEO_MARKET = b.GEO_MARKET
      )
      OR
      (
          a.REASON_CD = 'LOB'
          AND a.LOB_CD = b.LOB_CD
      )
      OR
      (
          a.REASON_CD = 'CAT'
          AND a.CTRCT_CAT_CD = b.CTRCT_CAT_CD
      )
      OR
      (
          a.REASON_CD = 'LOBST'
          AND a.LOB_CD = b.LOB_CD
          AND a.[derived_GEO_MARKET] = b.GEO_MARKET
      );