SQL - 对 SQL 查询执行 SQL 查询

Question

是否可以在另一个 SQL 查询上运行 SQL 查询？ 我有一个 SQL 查询，它使用 Union All 子句将许多列编译为一个。我需要使用 group 子句，但您会发现这是不可能的。有没有办法在这个上运行另一个 SQL 查询？

查询：

Select Ins1 as Insurance

From InsAuth2
WHERE Ins1 IS NOT NULL
Group By Ins1
Union All
Select Ins2 as Insurance
From InsAuth2
WHERE Ins2 IS NOT NULL
Group By Ins2
Union All
Select Ins3 as Insurance
From InsAuth2
WHERE Ins3 IS NOT NULL
Union All
Select Ins4 as Insurance
From InsAuth2
WHERE Ins4 IS NOT NULL
Union All
Select Ins5 as Insurance
From InsAuth2
WHERE Ins5 IS NOT NULL

我需要能够从这个已编译的列中选择唯一值。我可以在每个语句中使用 group by，但这只会从原始列返回唯一值。如果一个列的唯一值对另一列也是唯一的，则编译的列可能不具有所有唯一值。因此，我必须在这个查询上运行另一个 SQL 查询。有什么建议吗？

Answer 1

使用内部查询，即

SELECT *
FROM
(
    YourQueryHere
) Inn
GROUP BY YourColumnHere

Answer 2

使用派生表和 distinct：

select distinct Insurance
from
(
  Select Ins1 as Insurance
  From InsAuth2
  WHERE Ins1 IS NOT NULL
  Union All
  Select Ins2 as Insurance
  From InsAuth2
  WHERE Ins2 IS NOT NULL
  ...
) table_alias

Answer 3

如果您使用UNION 而不是UNION ALL，它将过滤掉重复项

Select Ins1 as Insurance
From InsAuth2
WHERE Ins1 IS NOT NULL
Group By Ins1
Union
Select Ins2 as Insurance
From InsAuth2
WHERE Ins2 IS NOT NULL
Group By Ins2
Union
Select Ins3 as Insurance
From InsAuth2
WHERE Ins3 IS NOT NULL
Union
Select Ins4 as Insurance
From InsAuth2
WHERE Ins4 IS NOT NULL
Union
Select Ins5 as Insurance
From InsAuth2
WHERE Ins5 IS NOT NULL

Answer 4

尝试使用 UNION 而不是 UNION ALL

谢谢