SQL：更改插入查询以容纳不同列的输入表

Question

给出2个临时表：

tmp

A  B
a1 b1 

tmp2

A  B  C
a2 b2 c2

然后，通过简单地将A，B和C列分别重命名为COL1，COL2和COL3，我创建了＃tmp3（来自#tmp）和＃tmp4（以及＃tmp2）。

我目前有：

Create table #tmp5 (
A varchar (100)
, B varchar (100
, C varchar (100)
)

Declare @column1 as varchar(256)
Declare @column2 as varchar(256)
Declare @column3 as varchar(256)

Select @column1 = column_name 
from tempdb.INFORMATION_SCHEMA.columns
where TABLE_NAME = #tmp2 and ORDINAL_POSITION = 1

Select @column2 = column_name 
from tempdb.INFORMATION_SCHEMA.columns
where TABLE_NAME = #tmp2 and ORDINAL_POSITION = 2

Select @column3 = column_name 
from tempdb.INFORMATION_SCHEMA.columns
where TABLE_NAME = #tmp2 and ORDINAL_POSITION = 3

Insert into #tmp5
    Select
    (Case when @column1 = 'A' then COL1
          when @column2 = 'A' then COL2
          when @column3 = 'A' then COL3 else NULL end) as A
    (Case when @column1 = 'B' then COL1
          when @column2 = 'B' then COL2
          when @column3 = 'B' then COL3 else NULL end) as B
    (Case when @column1 = 'C' then COL1
          when @column2 = 'C' then COL2
          when @column3 = 'C' then COL3 else NULL end) as C
    from #tmp4

我想运行以上查询，以便它对两种输入类型均有效（#tmp 2列和＃tmp2 3列）。当前，该代码仅适用于3列表，因为我收到一条错误消息，指出COL3无效。我将如何更改以适应两种类型？

我尝试使用if语句，当when语句尝试这种情况，但似乎没有在查询中使用COL3的情况... :(