Question

我对UNION条款有几个问题我想过滤一个字段的总结果。一个想法是过滤每个查询，但它可能是＆＃34;硬＆＃34;什么都不做。

SELECT 'GROUP1' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
UNION
SELECT 'GROUP2' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
UNION
SELECT 'GROUP3' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE

--WHERE LicenseCode > 7878

显然，最终的陈述并不奏效。如何过滤总UNION结果？（不要在每个查询中加上语句（WHERE LicenseCode > 7878）。

谢谢！此致

Answer 1

首先，请勿使用UNION，请使用UNION ALL。 UNION会产生额外的开销，用于删除重复项。

其次，你的问题的答案是使用子查询或CTE：

SELECT t.*
FROM ((SELECT 'GROUP1' AS GROUP, LicenseCode, Field2, Field3, Field4
       FROM TABLE
      ) UNION ALL
      (SELECT 'GROUP2' AS GROUP, LicenseCode, Field2, Field3, Field4
       FROM TABLE
      ) UNION ALL
      (SELECT 'GROUP3' AS GROUP, LicenseCode, Field2, Field3, Field4
       FROM TABLE
      )
     ) t
WHERE LicenseCode > 7878;

SQL Server有一个智能优化器。我很确定它将WHERE子句推送到各个子查询。这是一项重要的优化。在不这样做的数据库中，您需要重复WHERE条件。

注意：如果您从同一个表中进行选择，则有更好的方法来编写查询：

select grp, t.licensecode, t.field2, t.field3, t.field4
from table t cross apply
     (values ('group1'), ('group2'), ('group3')) g(grp)
where licensecode > 7878;

Answer 2

使联合成为子查询，然后过滤结果：

select group, LicenseCode, Field2, Field3, Field4
from (
  SELECT 'GROUP1' AS GROUP, LicenseCode, Field2, Field3, Field4
  FROM TABLE
  UNION
  SELECT 'GROUP2' AS GROUP, LicenseCode, Field2, Field3, Field4
  FROM TABLE
  UNION
  SELECT 'GROUP3' AS GROUP, LicenseCode, Field2, Field3, Field4
  FROM TABLE
) x
WHERE LicenseCode > 7878

Answer 3

如果您只想按任意顺序对结果进行排序，那么您只需要通过语句为联合中的最后一个选择进行排序。默认情况下，它会为您整理整个结果。所以看起来应该是这样的

SELECT 'GROUP1' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
UNION
SELECT 'GROUP2' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
UNION
SELECT 'GROUP3' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
ORDER BY YourColumnName

但是从这里开始，您还希望根据列过滤记录。请试试这个

;WITH CTE
AS
(
    SELECT 'GROUP1' AS GROUP, LicenseCode, Field2, Field3, Field4
    FROM TABLE
    UNION
    SELECT 'GROUP2' AS GROUP, LicenseCode, Field2, Field3, Field4
    FROM TABLE
    UNION
    SELECT 'GROUP3' AS GROUP, LicenseCode, Field2, Field3, Field4
    FROM TABLE
)
SELECT
    *
    FROM CTE
        WHERE LicenseCode > 7878
            ORDER BY Your Columnname

Answer 4

利用CTE。

; WITH CTE AS
(
SELECT 'GROUP1' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
UNION
SELECT 'GROUP2' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
UNION
SELECT 'GROUP3' AS GROUP, LicenseCode, Field2, Field3, Field4
FROM TABLE
)
select * from CTE WHERE LicenseCode > 7878

创建一个＆＃34; final＆＃34;在UNION SQL中过滤

4 个答案: