mysql练习
select * from tableName limit i,n
# tableName：表名
# i：为查询结果的索引值(默认从0开始)，当i=0时可省略i
# n：为查询结果返回的数量
# i与n之间使用英文逗号","隔开
#
limit n 等同于 limit 0,n
12345678
栗子：
# 查询10条数据，索引从0到9，第1条记录到第10条记录 select * from t_user limit 10; select * from t_user limit 0,10; # 查询8条数据，索引从5到12，第6条记录到第13条记录 select * from t_user limit 5,8;
1、查询男生、女生的人数；
select group_concat(sname),count(sid) from student group by gender; select count(sid) from student group by gender;
2、查询姓“张”的学生名单
select sname from student where sname like '张%';
3、课程平均分从高到低显示
select avg(num) from score group by course_id order by avg(num) desc;
4、查询有课程成绩小于60分的同学的学号、姓名；
select sid,sname from student where sid in (select student_id from score where num < 60); select stu.sid,stu.sname,s.num from student as stu inner join score as s on stu.sid = s.student_id where s.num < 60;
5、查询至少有一门课与学号为1的同学所学课程相同的同学的学号和姓名；
select sid,sname from student where sid in (select student_id from score where course_id in (select course_id from score where student_id = 1));
6、查询出只选修了一门课程的全部学生的学号和姓名；
查询时报错：Operand should contain 1 column(s)。
原因是因为in条件后面有多个字段，in后面只能有一个字段。
如果如下写法加了一个course_id 就会报错
select sid,sname from student where sid in (select student_id ，course_id from score group by student_id having count(course_id) = 1); select sid,sname from student where sid in (select student_id from score group by student_id having count(course_id) = 1);
7、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；
select * from (select max(num),min(num),course_id from score group by course_id) as sc inner join course as cc on sc.course_id = cc.cid;
8、查询课程编号“2”的成绩比课程编号“1”课程低的所有同学的学号、姓名；
自己的答案:
包括： student_num | sname | sid | student_id | course_id | num | cid | cname | teacher_id select * from (select * from (select student.sid as student_num,student.sname from student where
((select score.num from score where score.course_id ='1' and student.sid = score.student_id) >
(select score.num from score where score.course_id ='2' and student.sid = score.student_id)))
as stu inner join score as s on stu.student_num = s.student_id where s.course_id in (1,2)) as ss inner join course as cc on ss.course_id = cc.cid;
标准答案：
包括：student_num | sname select student.sid as student_num,student.sname from student where
((select score.num from score where score.course_id ='1' and student.sid = score.student_id) >
(select score.num from score where score.course_id ='2' and student.sid = score.student_id))
解析：通过select student.sid as student_num,student.sname from student
查出student表中所有的student.sid as student_num,student.sname，然后通过where中的两个子查询，
查出当前的student.sid = score.student_id 的学生的score.course_id ='1'的课程分数(num) > 当前的student.sid = score.student_id 的学生的score.course_id ='2'的课程分数(num) 的学号和姓名
9、查询“生物”课程比“物理”课程成绩高的所有学生的学号；
自己的答案:
select * from (select * from (select student.sid as student_num,student.sname from student where
((select score.num from score where score.course_id = (select cid from course where cname = '生物')
and student.sid = score.student_id) > (select score.num from score
where score.course_id = (select cid from course where cname = '物理') and student.sid = score.student_id)))
as stu inner join score as s on stu.student_num = s.student_id where s.course_id in (1,2)) as ss inner join course as cc on ss.course_id = cc.cid order by sname;
10、查询平均成绩大于60分的同学的学号和平均成绩;
select student_id,avg(num) from score group by student_id having avg(num) > 60;
11、查询所有同学的学号、姓名、选课数、总成绩；
自己的答案:
select * from (select stud.sid,stud.sname,ss.count_course from (select stu.sid,stu.sname,count(course_id) as count_course,s.num from student as stu left join score as s on stu.sid = s.student_id group by student_id) as ss right join student as stud on ss.sid = stud.sid) as a left join (select sum(num),student_id from score group by student_id) as b on a.sid = b.student_id;
标准答案：
select sid,sname, (select sum(num) from score sc where sc.student_id = s1.sid) as sum_score, (select count(course_Id) from score s2 where s2.student_Id = s1.sid) as count_course from student s1;
12、查询姓“李”的老师的个数；
select count(tid) from teacher where tname like '李%';
13、查询没学过“张磊老师”课的同学的学号、姓名；
select sid,sname from student where sid not in (select student_id from score where course_id in
(select cid from course where teacher_id in (select tid from teacher where tname = '张磊老师')));
14、查询学过“1”并且也学过编号“2”课程的同学的学号、姓名；
select sid,sname from student where sid in (select student_id from score where course_id in(1,2) group by student_id having count(student_id) = 2);
15、查询学过“李平老师”所教的所有课的同学的学号、姓名；
select sid,sname from student where sid in (select student_id from score where course_id in (select cid from course where teacher_id in (select tid from teacher where tname = '李平老师') ) );
