sql题

• 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

• 学生表 Student

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')

• 成绩表 SC

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)

• 课程表 Course

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')

• 教师表 Teacher

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 |
+------+--------+--------------+-----------+

• 查有成绩的学生信息

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 |
+-----------+--------------+--------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+