sql 横行变竖行 行 列转换

现在有一张表A 有三列  c1,c2,c3 ，只有一条记录,值分别为v1,v2,v3
A:
c1  c2    c3

v1  v2    v3

要把它转成临时表#t 只有两列m,n，其中m列放A表的列表，n列放A表的列对应的值

#t

  m  n
 
  c1  v1
  c2  v2
  c3  v3
请问怎么转换，请高手给出语句，急。。。。

---

SQL code:
select c1 as m,v1 as n from a
union all
select c2 as m,v2 as n from a
union all
select c3 as m,v3 as n from a

---

不是这样子的，c1,c2,c2是列名，要在#t里面有列名

---

SQL code:
--临时表不存在
select
*
into
#t
from
(
select c1 as m,v1 as n from a
union all
select c2 as m,v2 as n from a
union all
select c3 as m,v3 as n from a
)t


----临时表存在
insert into #t(m,n) select * from (
select c1 as m,v1 as n from a
union all
select c2 as m,v2 as n from a
union all
select c3 as m,v3 as n from a
)t

---

SQL code:

declare @A table (col1 varchar(2),col2 varchar(2),col3 varchar(2))
insert into @A
select 'c1','c2','c3' union all