数据库中有3个表 teacher表，student表，teacher_student关系表。
teacher表：teacher_id，name，age
student表：student_id，name，age
teacher_student表：teacher_id，student_id
答案一：
select count(*), ts.teacher_id
from teacher_student ts
left join teacher t on t.teacher_id = ts.teacher_id
left join student s on s.student_id = ts.student_id
where
t.age < 45
and s.age > 12
group by ts.teacher_id
答案二：
select count(*), ts.teacher_id
from teacher_student ts, teacher t, student s
where
t.teacher_id = ts.teacher_id
and s.student_id = ts.student_id
and t.age < 45
and s.age > 12
group by ts.teacher_id
