select t.* from haha t,haha tt
WHERE t.a=tt.a and t.b<>tt.b and
t.a in (
select haha.a from haha GROUP BY haha.a HAVING COUNT(*)=2 )

select * from lianxi where a not in(select a from lianxi group by a,b having count(a)=2)

你是需要哪一种sql语句，查询的吗

用group by试试············

先给个思路吧，先查询b统计结果等于2的作为临时表，在使用条件过滤两者不同。可参考楼上的SQL