一个表有f1,f2,f3,f4这些字段.我想把f2,f3,f4都追加到f1字段上.怎么写SQL?

CREATE TABLE [dbo].[a] (
[f1] [int],
[f2] [int],
[f3] [int],
[f4] [int]
)
GO

insert into a select 11 ,21,31,41
union all select 12 ,22,32,42
union all select 13 ,23,33,43

select * from a
--------------------
f1 f2 f3 f4
11 21 31 41
12 22 32 42
13 23 33 43

insert into a(f1) select f2 from a
union all select f3 from a
union all select f4 from a

select * from a
------------------------
f1 f2 f3 f4
11 21 31 41
12 22 32 42
13 23 33 43
21 NULL NULL NULL
22 NULL NULL NULL
23 NULL NULL NULL
31 NULL NULL NULL
32 NULL NULL NULL
33 NULL NULL NULL
41 NULL NULL NULL
42 NULL NULL NULL
43 NULL NULL NULL