- 用一条SQL 语句 查询出每门课都大于80 分的学生姓名
| name |
course |
grade |
| 张三 |
语文 |
81 |
| 张三 |
数学 |
75 |
| 李四 |
语文 |
76 |
| 李四 |
数学 |
90 |
| 王五 |
语文 |
81 |
| 王五 |
数学 |
100 |
| 王五 |
英语 |
90 |
select name from table group by name having min(grade) > 80
学生成绩例子
create table Student(Sid varchar(6), Sname varchar(10), Sage datetime, Ssex varchar(10));
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女')
create table SC(Sid varchar(10), Cid varchar(10), score decimal(18,1));
insert into SC values('01' , '01' , 80);
insert into SC values('01' , '02' , 90);
insert into SC values('01' , '03' , 99);
insert into SC values('02' , '01' , 70);
insert into SC values('02' , '02' , 60);
insert into SC values('02' , '03' , 80);
insert into SC values('03' , '01' , 80);
insert into SC values('03' , '02' , 80);
insert into SC values('03' , '03' , 80);
insert into SC values('04' , '01' , 50);
insert into SC values('04' , '02' , 30);
insert into SC values('04' , '03' , 20);
insert into SC values('05' , '01' , 76);
insert into SC values('05' , '02' , 87);
insert into SC values('06' , '01' , 31);
insert into SC values('06' , '03' , 34);
insert into SC values('07' , '02' , 89);
insert into SC values('07' , '03' , 98)
create table Course(Cid varchar(10),Cname varchar(10),Tid varchar(10));
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03')
create table Teacher(Tid varchar(10),Tname varchar(10));
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五')
image.png
- 查询" 01 “课程比” 02 "课程成绩高的学生的信息及课程分数
select s.*, a.score as score_01, b.score as score_02
from student s,
(select sid, score from sc where cid=01) a,
(select sid, score from sc where cid=02) b
where a.sid = b.sid and a.score > b.score and s.sid = a.sid
+------+--------+---------------------+------+----------+----------+
| Sid | Sname | Sage | Ssex | score_01 | score_02 |
+------+--------+---------------------+------+----------+----------+
| 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 70.0 | 60.0 |
| 04 | 李云 | 1990-08-06 00:00:00 | 男 | 50.0 | 30.0 |
+------+--------+---------------------+------+----------+----------+
- 查询平均成绩大于等于 60 分的同学的学生编号和学生姓名和平均成绩
select s.sid, sname, avg(score) as avg_score
from student as s, sc
where s.sid = sc.sid
group by s.sid
having avg_score > 60
- 没有group by s.sid 就是返回平均数,并且因为有avg函数，只返回一条数据
+------+--------+-----------+
| sid | sname | avg_score |
+------+--------+-----------+
| 01 | 赵雷 | 89.66667 |
| 02 | 钱电 | 70.00000 |
| 03 | 孙风 | 80.00000 |
| 05 | 周梅 | 81.50000 |
| 07 | 郑竹 | 93.50000 |
+------+--------+-----------+
-
查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩(没成绩的显示为 null )
- 这道题得用到left join或者right join，不能用where连接，因为题目说了要求有显示为null的，where是inner join，不会出现null，在这道题里会查不出第08号学生。
select s.sid, s.sname, count(cid) as 选课总数, sum(score) as 总成绩
from student as s left join sc
on s.sid = sc.sid
group by s.sid
+------+--------+--------------+-----------+
| sid | sname | 选课总数 | 总成绩 |
+------+--------+--------------+-----------+
| 01 | 赵雷 | 3 | 269.0 |
| 02 | 钱电 | 3 | 210.0 |
| 03 | 孙风 | 3 | 240.0 |
| 04 | 李云 | 3 | 100.0 |
| 05 | 周梅 | 2 | 163.0 |
| 06 | 吴兰 | 2 | 65.0 |
| 07 | 郑竹 | 2 | 187.0 |
| 08 | 王菊 | 0 | NULL |
+------+--------+--------------+-----------+
select s.sid, s.sname, count(*) as 选课总数, sum(score) as 总成绩,
sum(case when cid = 01 then score else null end) as score_01,
sum(case when cid = 02 then score else null end) as score_02,
sum(case when cid = 03 then score else null end) as score_03
from student as s, sc
where s.sid = sc.sid
group by s.sid
+------+--------+--------------+-----------+----------+----------+----------+
| sid | sname | 选课总数 | 总成绩 | score_01 | score_02 | score_03 |
+------+--------+--------------+-----------+----------+----------+----------+
| 01 | 赵雷 | 3 | 269.0 | 80.0 | 90.0 | 99.0 |
| 02 | 钱电 | 3 | 210.0 | 70.0 | 60.0 | 80.0 |
| 03 | 孙风 | 3 | 240.0 | 80.0 | 80.0 | 80.0 |
| 04 | 李云 | 3 | 100.0 | 50.0 | 30.0 | 20.0 |
| 05 | 周梅 | 2 | 163.0 | 76.0 | 87.0 | NULL |
| 06 | 吴兰 | 2 | 65.0 | 31.0 | NULL | 34.0 |
| 07 | 郑竹 | 2 | 187.0 | NULL | 89.0 | 98.0 |
+------+--------+--------------+-----------+----------+----------+----------+
- score没有加sum,哪个score加了那个生效
image.png
- 查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
- having虽然是在select后影响的。但并不意味着having只能操作select出来的字段。
SELECT s.sid,s.sname ,AVG(sc.`score`)
FROM student s,sc
WHERE s.sid = sc.sid AND s.sid IN(
SELECT sc2.sid
FROM sc sc2
WHERE sc2.`score`<60
GROUP BY sc2.sid
HAVING COUNT(sc2.score)>=2
)
GROUP BY s.sid
- 查询各科成绩最高分、最低分和平均分，以如下形式显示：课程 ID，课程 name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率(及格为>=60，中等为：70-80，优良为：80-90，优秀为：>=90）。要求输出课程号和选修人数，查询结果按人数降序排列，若人数相同，按课程号升序排列。
select c.cid as 课程号, c.cname as 课程名称, count(*) as 选修人数,
max(score) as 最高分, min(score) as 最低分, avg(score) as 平均分,
sum(case when score >= 60 then 1 else 0 end)/count(*) as 及格率,
sum(case when score >= 70 and score < 80 then 1 else 0 end)/count(*) as 中等率,
sum(case when score >= 80 and score < 90 then 1 else 0 end)/count(*) as 优良率,
sum(case when score >= 90 then 1 else 0 end)/count(*) as 优秀率
from sc, course c
where c.cid = sc.cid
group by c.cid
order by count(*) desc, c.cid asc
+-----------+--------------+--------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| 课程号 | 课程名称 | 选修人数 | 最高分 | 最低分 | 平均分 | 及格率 | 中等率 | 优良率 | 优秀率 |
+-----------+--------------+--------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| 01 | 语文 | 6 | 80.0 | 31.0 | 64.50000 | 0.6667 | 0.3333 | 0.3333 | 0.0000 |
| 02 | 数学 | 6 | 90.0 | 30.0 | 72.66667 | 0.8333 | 0.0000 | 0.5000 | 0.1667 |
| 03 | 英语 | 6 | 99.0 | 20.0 | 68.50000 | 0.6667 | 0.0000 | 0.3333 | 0.3333 |
+-----------+--------------+--------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
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