一、查询数据
二、问题
21.查询男生、女生人数
mysql> select ssex, count(*) from student
-> group by ssex;
+------+----------+
| ssex | count(*) |
+------+----------+
| 女 | 8 |
| 男 | 4 |
+------+----------+
2 rows in set (0.02 sec)
22.查询名字中含有「风」字的学生信息
mysql> select *
-> from student
-> where student.Sname like '%风%'
-> ;
+------+--------+---------------------+------+
| SId | Sname | Sage | Ssex |
+------+--------+---------------------+------+
| 03 | 孙风 | 1990-12-20 00:00:00 | 男 |
+------+--------+---------------------+------+
1 row in set (0.00 sec)
23.查询同名同性学生名单，并统计同名人数
mysql> select sname, count(*) from student
-> group by sname
-> having count(*)>1;
+--------+----------+
| sname | count(*) |
+--------+----------+
| 李四 | 2 |
+--------+----------+
1 row in set (0.00 sec)
24.查询 1990 年出生的学生名单
mysql> select *
-> from student
-> where YEAR(student.Sage)=1990;
+------+--------+---------------------+------+
| SId | Sname | Sage | Ssex |
+------+--------+---------------------+------+
| 01 | 赵雷 | 1990-01-01 00:00:00 | 男 |
| 02 | 钱电 | 1990-12-21 00:00:00 | 男 |
| 03 | 孙风 | 1990-12-20 00:00:00 | 男 |
| 04 | 李云 | 1990-12-06 00:00:00 | 男 |
+------+--------+---------------------+------+
4 rows in set (0.00 sec)
25.查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
mysql> select sc.CId,avg(sc.score) from sc,course
-> group by sc.cid
-> order by avg(sc.score) desc,cid asc;
+------+---------------+
| CId | avg(sc.score) |
+------+---------------+
| 02 | 72.66667 |
| 03 | 68.50000 |
| 01 | 64.50000 |
+------+---------------+
3 rows in set (0.00 sec)
26.查询平均成绩大于等于 85 的所有学生的学号、姓名和平均成绩
mysql> select student.sid, AVG(sc.score) as aver from student, sc
-> where student.sid = sc.sid
-> group by sc.sid
-> having aver > 85;
+------+----------+
| sid | aver |
+------+----------+
| 01 | 89.66667 |
| 07 | 93.50000 |
+------+----------+
2 rows in set (0.00 sec)
27.查询课程名称为「数学」，且分数低于 60 的学生姓名和分数
mysql> select student.sname, sc.score from student, sc, course
-> where student.sid = sc.sid
-> and course.cid = sc.cid
-> and course.cname = "数学"
-> and sc.score < 60;
+--------+-------+
| sname | score |
+--------+-------+
| 李云 | 30.0 |
+--------+-------+
1 row in set (0.00 sec)
28.查询所有学生的课程及分数情况（存在学生没成绩，没选课的情况）
mysql> select student.sname, cid, score from student
-> left join sc
-> on student.sid = sc.sid;
+--------+------+-------+
| sname | cid | score |
+--------+------+-------+
| 赵雷 | 01 | 80.0 |
| 赵雷 | 02 | 90.0 |
| 赵雷 | 03 | 99.0 |
| 钱电 | 01 | 70.0 |
| 钱电 | 02 | 60.0 |
| 钱电 | 03 | 80.0 |
| 孙风 | 01 | 80.0 |
| 孙风 | 02 | 80.0 |
| 孙风 | 03 | 80.0 |
| 李云 | 01 | 50.0 |
| 李云 | 02 | 30.0 |
| 李云 | 03 | 20.0 |
| 周梅 | 01 | 76.0 |
| 周梅 | 02 | 87.0 |
| 吴兰 | 01 | 31.0 |
| 吴兰 | 03 | 34.0 |
| 郑竹 | 02 | 89.0 |
| 郑竹 | 03 | 98.0 |
| 张三 | NULL | NULL |
| 李四 | NULL | NULL |
| 李四 | NULL | NULL |
| 赵六 | NULL | NULL |
| 孙七 | NULL | NULL |
+--------+------+-------+
23 rows in set (0.00 sec)
29查询任何一门课程成绩在 70 分以上的姓名、课程名称和分数
mysql> select student.sname, course.cname,sc.score from student,course,sc
-> where sc.score>70
-> and student.sid = sc.sid
-> and sc.cid = course.cid;
+--------+--------+-------+
| sname | cname | score |
+--------+--------+-------+
| 赵雷 | 语文 | 80.0 |
| 赵雷 | 数学 | 90.0 |
| 赵雷 | 英语 | 99.0 |
| 钱电 | 英语 | 80.0 |
| 孙风 | 语文 | 80.0 |
| 孙风 | 数学 | 80.0 |
| 孙风 | 英语 | 80.0 |
| 周梅 | 语文 | 76.0 |
| 周梅 | 数学 | 87.0 |
| 郑竹 | 数学 | 89.0 |
| 郑竹 | 英语 | 98.0 |
+--------+--------+-------+
11 rows in set (0.00 sec)
30.查询不及格的课程
mysql> select DISTINCT sc.CId
-> from sc
-> where sc.score <60;
+------+
| CId |
+------+
| 01 |
| 02 |
| 03 |
+------+
3 rows in set (0.00 sec)