tj11:

​
select *
from t_student
where grade = '大一' and major = '软件工程' ​

tj12:

SELECTt_student.`name`, count(t_choice.cid)FROMt_choiceINNER JOINt_courseON t_choice.cid = t_course.idINNER JOINt_studentON t_choice.sid = t_student.id
GROUP BYt_choice.sid
HAVINGcount(t_choice.cid) < 3

tj13:

select t_student.id,t_student.`name`,t_student.grade
from t_student join t_choice on t_student.id=t_choice.sid
join t_course on t_choice.cid = t_course.id
where t_course.name='网页设计'