题目：有一张老师表T，字段是T_ID，T_NAME；有一张学生表S，字段是S_ID，S_NAME；还有一张班级表C，字段是T_ID，S_ID，C_NAME，其中C_NAME的取值只有‘大班’和‘小班’，请查询出符合条件的老师的名字，条件是老师在大班中带的学生数大于老师在小班中带的学生数
--建立T表
create table T( t_id number(4) primary key, t_name varchar2(20));
--建立S表
create table S( s_id number(4) primary key, s_name varchar2(20));
--建立C表
create table C( t_id number(4) references T(t_id), s_id number(4) references S(s_id), c_name char(6) check(c_name in (‘大班‘,‘小班‘)));
--插入数据
--T表 insert into T values(1,‘A‘); insert into T values(2,‘B‘); --S表 insert into S values(11,‘1A‘); insert into S values(12,‘1B‘); insert into S values(13,‘1C‘); insert into S values(14,‘1D‘); insert into S values(15,‘1E‘); insert into S values(16,‘1F‘); insert into S values(17,‘1G‘); insert into S values(18,‘1H‘); --C表 insert into C values(1,11,‘大班‘); insert into C values(1,12,‘大班‘); insert into C values(1,17,‘大班‘); insert into C values(1,16,‘大班‘); insert into C values(1,13,‘小班‘); insert into C values(1,14,‘小班‘); insert into C values(1,15,‘小班‘); insert into C values(2,18,‘小班‘);
--SQL语句
select t_name from t where t_id in ( select distinct c1.t_id from c c1 where (select count(*) from c c2 where c2.t_id = c1.t_id and c2.c_name=‘大班‘) >(select count(*) from c c3 where c3.t_id = c1.t_id and c3.c_name=‘小班‘))
--查询结果
T_NAME -------------------- A
原文：http://www.cnblogs.com/xkk112/p/4764118.html