联盟应该在另一个选择

Question

我想在3个不同的表上使用UNION ALL，使用SELECT INTO将它们合并到一个表中。

表1,2和3分别有15,7和8列。

那么，有没有办法让我使用UNION ALL并将表2和3默认为缺少列的NULL，而不是单独对它们进行分类？

例如，我一直在做：

SELECT  NULL as [Company_Code], NULL as [Doc_Code], 
NULL as [Doc_Type], [H ID] as [Document_No] FROM [table_2] 
INTO BIG_TABLE
UNION ALL 
SELECT
[Document Company] as [Company_Code], [Document Company] as [Doc_Code], 
[Doc Type] as [Doc_Type], NULL as [Document_No]
FROM [table_3]

通过这种方式，列数匹配，我可以UNION它们。

但是，我想知道是否有办法避免繁琐的机制来避免为每个丢失的列插入NULL，并且一次性自动完成？

感谢。

Answer 1

简而言之，没有。 Union结果集必须具有相同的数字/数据类型的列。如果你想让其余的集合填充null，最简单的方法就是这样做 -

select col1
, col2
, col3
, col4
from tbl1

union all

select null as col1
, null as col2
, null as col3
, null as col4
from tbl2

Answer 2

联盟应该在另一个选择

内

SELECT * FROM(
    SELECT col1, col2 FROM test_table1
  UNION ALL
    SELECT col1, col2,col3 FROM test_table2
);

结果将是col1，col2将跳过未匹配的列

Answer 3

使缺少的列无效的最佳方法是使用 null as column_name ，您要为其查看空列，如下所示：...

    SELECT id as var1, mgr_id as var2,name as var3 into exp_test1 FROM
 emp123    UNION ALL
      SELECT null as employeeid, null as departmentid,null as lastname FROM employee

Answer 4

我仍然建议使用@iliketocode。

动态列根据您的要求生成

这只是另一种选择，但纯粹是动态的（只需要一些时间来执行而不是上面的答案）。要在不定义每列的情况下实现结果，您可以创建动态查询，通过循环自动将列添加为null。

我面临同样的问题，由于时间短，我完成了上述工作。但是今天我填写了正确的解决方案（我当时没做过），我根据你的需要为你创建了一个样本，希望这对你有所帮助。

--create table t1 (col1 int , col2 int, col3 int)
--create table t2 (col1 int , col2 int, col3 int, col4 int)

--insert into t1 values (1,11,111), (2,22,222)
--insert into t2 values (1,11,111,1111), (2,22,222,null)

--Step 1 - Declaration of variable
Declare @NoOfColumnForUnion int = 5

declare @T1TableColumnList nvarchar(max) ='' ,@T2TableColumnList nvarchar(max) ='' , @colName nvarchar(500) , @test cursor

--Step 2 - Get the column list of first table i.e. t1 and store into @T1TableColumnList variable
set @test = cursor for 
                select name from syscolumns 
                where id = object_id('t1')                 

open @test
fetch next from @test into @colName
while @@fetch_status = 0 
begin
    set @T1TableColumnList = @T1TableColumnList + @colName + ','

    fetch next from @test into @colName 
end

set @T1TableColumnList = left( @T1TableColumnList , len(@T1TableColumnList )-1)

close @test
deallocate @test

--Step 3 - Get the column list of Second table i.e. t2 and store into @T2TableColumnList variable
set @test = cursor for 
                select name from syscolumns 
                where id = object_id('t2')                 

open @test
fetch next from @test into @colName
while @@fetch_status = 0 
begin
    set @T2TableColumnList = @T2TableColumnList + @colName + ','

    fetch next from @test into @colName 
end

set @T2TableColumnList = left( @T2TableColumnList , len(@T2TableColumnList )-1)

close @test
deallocate @test

--Step 4 - Check the length of column list to add null columns or remove columns
--First table check
Declare @T1lengthofColumnList int
set @T1lengthofColumnList =  (len(@T1TableColumnList) - len(replace(@T1TableColumnList, ',', '')) ) + 1

--add columns
if( @T1lengthofColumnList  < @NoOfColumnForUnion)
 Begin  
    While (@T1lengthofColumnList  < @NoOfColumnForUnion)
     Begin
        set @T1lengthofColumnList = @T1lengthofColumnList + 1
        Set @T1TableColumnList = @T1TableColumnList +  ', null col' + cast( @T1lengthofColumnList as varchar(10))
     End
 End
--remove columns
Else if( @T1lengthofColumnList  > @NoOfColumnForUnion)
 Begin
    While (@T1lengthofColumnList  > @NoOfColumnForUnion)
     Begin
        set @T1lengthofColumnList = @T1lengthofColumnList - 1       
        Set @T1TableColumnList = LEFT(@T1TableColumnList, LEN(@T1TableColumnList) - CHARINDEX(',',REVERSE(@T1TableColumnList))) 
     End
 End

--Second table check
Declare  @T2lengthofColumnList int
set @T2lengthofColumnList =  (len(@T2TableColumnList) - len(replace(@T2TableColumnList, ',', '')) ) + 1

--add columns
if( @T2lengthofColumnList  < @NoOfColumnForUnion)
 Begin  
    While (@T2lengthofColumnList  < @NoOfColumnForUnion)
     Begin
        set @T2lengthofColumnList = @T2lengthofColumnList + 1
        Set @T2TableColumnList = @T2TableColumnList +  ', null col' + cast( @T2lengthofColumnList as varchar(10))
     End
 End
--remove columns
Else if( @T2lengthofColumnList  > @NoOfColumnForUnion)
 Begin
    While (@T2lengthofColumnList  > @NoOfColumnForUnion)
     Begin
        set @T2lengthofColumnList = @T2lengthofColumnList - 1       
        Set @T2TableColumnList = LEFT(@T2TableColumnList, LEN(@T2TableColumnList) - CHARINDEX(',',REVERSE(@T2TableColumnList))) 
     End
 End

--Step 5 - create dynamic query and execute
DECLARE @template AS varchar(max)

SET @template = 'select ' + @T1TableColumnList + '  from t1 union all ' 
+ ' select ' + @T2TableColumnList + '  from t2 '

select @template 

EXEC (@template)


--drop table t1
--drop table t2

UNION ALL在3个表中，每个表中的列数不同 - 如何有效地执行此操作

4 个答案:

联盟应该在另一个选择

动态列根据您的要求生成