select case when rand() between 0 and 0.3 then b
        when rand() between 0.3 and 0.6 then e
        else g end 
from 你的表
where a=2;

------解决方案--------------------

这个是取随机column
create table #a(a int , b varchar(10),c varchar(10),d varchar(10),e varchar(10),f varchar(10),g varchar(10))

insert into #a
  select  1 ,'fd' ,'f34' ,'54a-' ,'dft4' ,'gh5_g' ,'dfttg'  union all
  select  2, 'tewrt', 't5e', 'ret4', 'hrt' ,'gh+', 'dfs3'   union all
  select  3, 'dtr', 'fsd', 'erw3', 'fs' ,'+_dsf' ,'sdg'

--select * from #a

declare @t int  
select @t=cast(RAND()*3 as int)
select (case @t when 0 then b when 1 then e  when 2 then g  end )x  from #a where a=2