tj11:

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

​

tj12:

SELECT
	t_student.`name`, 
	count(t_choice.cid)

FROM
	t_choice
	INNER JOIN
	t_course
	ON 
		t_choice.cid = t_course.id
	INNER JOIN
	t_student
	ON 
		t_choice.sid = t_student.id
GROUP BY
	t_choice.sid
HAVING
	count(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'