UNION ALL on 3 tables with different number of columns in each table - 如何有效地做到这一点

Question

我想在 3 个不同的 table 上使用 UNION ALL，然后使用 SELECT INTO.

将它们合并为一个 table

Table 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 并让 table 2 和 3 默认为缺失列的 NULL，而无需单独将它们分类？

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]

这样，列数就匹配了，我可以UNION了。

但是，我想知道是否有一种方法可以避免繁琐的机制来避免为每个丢失的列插入 NULL，并一次性自动完成？

谢谢。

Answer 1

Union 应该在另一个里面 select

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

结果将是 col1 和 col2，跳过不匹配的列

Answer 2

简而言之，没有。 Union合并的结果集必须具有相同的列数/数据类型。如果你想让剩余的集合填充null，最简单的方法就是做这样的事情-

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 3

使丢失的列无效的最佳方法是使用 null 作为 column_name，您希望看到的空列如下所示：。 ..

    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，但完全是动态的（只是需要一些时间来执行，而不是上面的答案）。要在不定义每一列的情况下获得结果，您可以创建动态查询，通过循环自动将列添加为 null。

我遇到同样的问题，由于时间短，我按照上面的方法完成了。但是今天我填写了正确的解决方案（当时我没有这样做）并且我根据需要为您创建了一个示例，希望这对您有所帮助。

--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 on 3 tables with different number of columns in each table - 如何有效地做到这一点

UNION ALL on 3 tables with different number of columns in each table - how to do so efficiently

tsql

sql-server

select-into

union-all

Union 应该在另一个里面 select

动态列根据您的要求生成