create table tb(id int, subjectid int, content varchar(10) , isright int)
insert into tb values(1 , 101 , '题目内容', 0 )
insert into tb values(2 , 101 , '题目内容', 0 )
insert into tb values(3 , 101 , '题目内容', 1 )
insert into tb values(4 , 101 , '题目内容', 1 )
insert into tb values(5 , 102 , '题目内容', 0 )
insert into tb values(6 , 102 , '题目内容', 1 )
insert into tb values(7 , 103 , '题目内容', 0 )
insert into tb values(8 , 103 , '题目内容', 1 )
insert into tb values(9 , 103 , '题目内容', 1 )
go

select subjectid,
  错误率 = cast(cast(sum(case isright when 0 then 1 else 0 end ) * 100.0 / (select count(1) from tb where subjectid = t.subjectid) as decimal(18,2)) as varchar) + '%'
from tb t
group by subjectid

drop table tb

/*
subjectid   错误率                             
----------- ------------------------------- 
101         50.00%
102         50.00%
103         33.33%

（所影响的行数为 3 行）

*/

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

 select subjectid,content,count(*) as 题目数量,
        sum(case when isright=0 then 1 else 0 end) as 错误数量，
         cast(sum(case when isright=0 then 1 else 0 end)/count(*)*100 as char(2))+'%' as 错误率
 from tb
 group by subjectid,content
 order by sum(case when isright=0 then 1 else 0 end) desc

------解决方案--------------------
SQL code
declare @t table(id int,subjectid int,[content] varchar(10), isright bit)
insert @t select 1,101,'题目内容',0
insert @t select 2,101,'题目内容',0
insert @t select 3,101,'题目内容',1
insert @t select 4,101,'题目内容',1
insert @t select 5,102,'题目内容',0
insert @t select 6,102,'题目内容',1
insert @t select 7,103,'题目内容',0
insert @t select 8,103,'题目内容',1
insert @t select 9,103,'题目内容',1

select subjectid,[content],count(1) 题目出现总数,sum(1-isright) 每题出错个数
from @t
group by subjectid,content
order by subjectid

subjectid   content    题目出现总数      每题出错个数
----------- ---------- ----------- -----------
101         题目内容       4           2
102         题目内容       2           1
103         题目内容       3           1

(3 行受影响)

------解决方案--------------------
SQL code
create table tb(id int, subjectid int, content varchar(10) , isright int)
insert into tb values(1 , 101 , '题目内容', 0 )
insert into tb values(2 , 101 , '题目内容', 0 )
insert into tb values(3 , 101 , '题目内容', 1 )
insert into tb values(4 , 101 , '题目内容', 1 )
insert into tb values(5 , 102 , '题目内容', 0 )
insert into tb values(6 , 102 , '题目内容', 1 )
insert into tb values(7 , 103 , '题目内容', 0 )
insert into tb values(8 , 103 , '题目内容', 1 )
insert into tb values(9 , 103 , '题目内容', 1 )
go

select subjectid,
sum(case isright when 0 then 1 else 0 end)'错误数]',
count(1) '总题目数',
cast(sum(case isright when 0 then 1 else 0 end ) * 100.0 / (select count(1) from tb where subjectid = t.subjectid) as decimal(18,2)) '错误率'
from tb