create table T( id int,name varchar(4))
insert into T values(1,'A'),(1,'B'),(2,'A'),(2,'C'),(3,'C');
--选出name有Ａ无Ｂ的id号
--方法一：
select * from (select * from T where name='A')as a where ID<>(select id from T where name='B')

create table T( id int,name varchar(4))
insert into T values(1,'A'),(1,'B'),(2,'A'),(2,'C'),(3,'C');


select t1.*
from T t1
where t1.name='A' and not exists
(select 1 from T t2 
where t2.id=t1.id and t2.name='B')

/*
id          name
----------- ----
2           A

(1 row(s) affected)
*/

------解决方案--------------------
SQL code
SELECT  *
FROM    t AS a
WHERE   NAME = 'a'
        AND NOT EXISTS ( SELECT 1
                         FROM   t
                         WHERE  id = a.id
                                AND NAME = 'b' )