pid     pname
 1       AA
 2       AA
 3       BB
 4       AA
 5       BB

Pname   per
AA      60%
BB      40%

SELECT  
  PName,
  CAST(COUNT(1)*1.0/(select count(1) from tb) AS DEC(18,0))AS T
FROM TB
GROUP BY PNAME

------解决方案--------------------
探讨

SQL code
SELECT  
  PName,
  CAST(COUNT(1)*1.0/(select count(1) from tb) AS DEC(18,0))AS T
FROM TB
GROUP BY PNAME


------解决方案--------------------
SQL code

--测试数据
create table #a 
(
    pid int 
,pname varchar(10)
)
insert into #a 
select 1,'aa' union all
select 2,'aa' union all
select 3,'bb' union all
select 4,'aa' union all
select 5 ,'bb' 

--表达式
declare @sql varchar(800)
set @sql ='select pname ,convert(varchar(10),cast(count(1)*100/(select count(1)from #a) as decimal(18,2)))+''%'' as per from #a group by pname'
print @sql
exec(@sql)
--结果：
pname      per
---------- -----------
aa         60.00%
bb         40.00%