Mysql实战50例

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最新推荐文章于 2025-05-14 21:33:30 发布

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最近为复习SQL语言，上网找了些实战例子(基于Mysql)练练手，部分例子给出了2种实现方式
一、建库、建表

# 创建数据库
create database mysql_exercise_50;
use mysql_exercise_50;

# 学生表
CREATE TABLE Student(
s_id VARCHAR(20),
s_name VARCHAR(20) NOT NULL DEFAULT '',
s_birth VARCHAR(20) NOT NULL DEFAULT '',
s_sex VARCHAR(10) NOT NULL DEFAULT '',
PRIMARY KEY(s_id)
);

# 课程表
CREATE TABLE Course(
c_id VARCHAR(20),
c_name VARCHAR(20) NOT NULL DEFAULT '',
t_id VARCHAR(20) NOT NULL,
PRIMARY KEY(c_id)
);

# 教师表
CREATE TABLE Teacher(
t_id VARCHAR(20),
t_name VARCHAR(20) NOT NULL DEFAULT '',
PRIMARY KEY(t_id)
);

# 成绩表
CREATE TABLE Score(
s_id VARCHAR(20),
c_id VARCHAR(20),
s_score INT(3),
PRIMARY KEY(s_id,c_id)
);

二、插入数据

# 插入学生表测试数据
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');
Student表：
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
| 05   | 周梅   | 1991-12-01 | 女    |
| 06   | 吴兰   | 1992-03-01 | 女    |
| 07   | 郑竹   | 1989-07-01 | 女    |
| 08   | 王菊   | 1990-01-20 | 女    |
+------+--------+------------+-------+

# 课程表测试数据
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');
Course表：
+------+--------+------+
| c_id | c_name | t_id |
+------+--------+------+
| 01   | 语文   | 02   |
| 02   | 数学   | 01   |
| 03   | 英语   | 03   |
+------+--------+------+

# 教师表测试数据
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
Teacher表：
+------+--------+
| t_id | t_name |
+------+--------+
| 01   | 张三   |
| 02   | 李四   |
| 03   | 王五   |
+------+--------+

# 成绩表测试数据
insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);
Score表：
+------+------+---------+
| s_id | c_id | s_score |
+------+------+---------+
| 01   | 01   |      80 |
| 01   | 02   |      90 |
| 01   | 03   |      99 |
| 02   | 01   |      70 |
| 02   | 02   |      60 |
| 02   | 03   |      80 |
| 03   | 01   |      80 |
| 03   | 02   |      80 |
| 03   | 03   |      80 |
| 04   | 01   |      50 |
| 04   | 02   |      30 |
| 04   | 03   |      20 |
| 05   | 01   |      76 |
| 05   | 02   |      87 |
| 06   | 01   |      31 |
| 06   | 03   |      34 |
| 07   | 02   |      89 |
| 07   | 03   |      98 |
+------+------+---------+

三、实战50例

# 1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数 
select st.*,sc.s_score as '语文' ,sc2.s_score '数学' 
from student st
left join score sc on sc.s_id=st.s_id and sc.c_id='01' 
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id='02'  
where sc.s_score>sc2.s_score
+------+--------+------------+-------+------+------+
| s_id | s_name | s_birth    | s_sex | 语文 | 数学 |
+------+--------+------------+-------+------+------+
| 02   | 钱电   | 1990-12-21 | 男    |   70 |   60 |
| 04   | 李云   | 1990-08-06 | 男    |   50 |   30 |
+------+--------+------------+-------+------+------+

# 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
select st.*,sc.s_score '语文',sc2.s_score '数学' from student st
left join score sc on sc.s_id=st.s_id and sc.c_id='01'
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id='02'
where sc.s_score<sc2.s_score
+------+--------+------------+-------+------+------+
| s_id | s_name | s_birth    | s_sex | 语文 | 数学 |
+------+--------+------------+-------+------+------+
| 01   | 赵雷   | 1990-01-01 | 男    |   80 |   90 |
| 05   | 周梅   | 1991-12-01 | 女    |   76 |   87 |
+------+--------+------------+-------+------+------+

# 3、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 st.s_id,st.s_name,ROUND(AVG(sc.s_score),2) cjScore from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id having AVG(sc.s_score)>=60
+------+--------+---------+
| s_id | s_name | cjScore |
+------+--------+---------+
| 01   | 赵雷   |   89.67 |
| 02   | 钱电   |   70.00 |
| 03   | 孙风   |   80.00 |
| 05   | 周梅   |   81.50 |
| 07   | 郑竹   |   93.50 |
+------+--------+---------+

# 4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩(包括有成绩的和无成绩的)
select st.s_id,st.s_name,(case when ROUND(AVG(sc.s_score),2) is null then 0 else ROUND(AVG(sc.s_score)) end ) cjScore from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id having AVG(sc.s_score)<60 or AVG(sc.s_score) is NULL
+------+--------+---------+
| s_id | s_name | cjScore |
+------+--------+---------+
| 04   | 李云   |      33 |
| 06   | 吴兰   |      33 |
| 08   | 王菊   |       0 |
+------+--------+---------+

# 5、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
select st.s_id,st.s_name,count(sc.c_id) '选课总数',(case when sum(sc.s_score) is null or sum(sc.s_score)='' then 0 else sum(sc.s_score) end) '总成绩'
from student st
left outer join score sc on st.s_id=sc.s_id
group by st.s_id;
+------+--------+----------+--------+
| s_id | s_name | 选课总数 | 总成绩 |
+------+--------+----------+--------+
| 01   | 赵雷   |        3 |    269 |
| 02   | 钱电   |        3 |    210 |
| 03   | 孙风   |        3 |    240 |
| 04   | 李云   |        3 |    100 |
| 05   | 周梅   |        2 |    163 |
| 06   | 吴兰   |        2 |     65 |
| 07   | 郑竹   |        2 |    187 |
| 08   | 王菊   |        0 |      0 |
+------+--------+----------+--------+

# 6、查询"李"姓老师的数量 
select t.t_name,count(t.t_id) from teacher t
group by t.t_id having t.t_name like "李%"; 
+--------+----------+
| t_name | count(*) |
+--------+----------+
| 李四   |        1 |
+--------+----------+

# 7、查询学过"张三"老师授课的同学的信息 
select st.* from student st 
left join score sc on sc.s_id=st.s_id
left join course c on c.c_id=sc.c_id
left join teacher t on t.t_id=c.t_id
 where t.t_name="张三"
 +------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
| 05   | 周梅   | 1991-12-01 | 女    |
| 07   | 郑竹   | 1989-07-01 | 女    |
+------+--------+------------+-------+

# *8、查询没学过"张三"老师授课的同学的信息 
-- 张三老师教的课
select c.* from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三"
-- 有张三老师课成绩的st.s_id
 select sc.s_id from score sc where sc.c_id in (select c.c_id from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三")
-- 不在上面查到的st.s_id的学生信息,即没学过张三老师授课的同学信息
 select st.* from student st where st.s_id not in(
  select sc.s_id from score sc where sc.c_id in (select c.c_id from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三")
 )
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 06   | 吴兰   | 1992-03-01 | 女    |
| 08   | 王菊   | 1990-01-20 | 女    |
+------+--------+------------+-------+

# *9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
select st.* from student st 
inner join score sc on sc.s_id = st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
where st.s_id in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id
inner join course c2 on c2.c_id=sc2.c_id and c2.c_id="02"
)
或
select st.*
from student st 
inner join score s on st.s_id = s.s_id and s.c_id = '01'
inner join score s2 on st.s_id=s2.s_id and s2.c_id='02';
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
| 05   | 周梅   | 1991-12-01 | 女    |
+------+--------+------------+-------+

# 10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
select st.* from student st 
inner join score sc on sc.s_id = st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
where st.s_id not in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id
inner join course c2 on c2.c_id=sc2.c_id and c2.c_id="02"
)
或
select st.* from student st 
inner join score sc on sc.s_id = st.s_id and sc.c_id="01"
where st.s_id not in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id and sc2.c_id="02"
);
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 06   | 吴兰   | 1992-03-01 | 女    |
+------+--------+------------+-------+

# 11、查询没有学全所有课程的同学的信息
select * from student where s_id not in (
select st.s_id from student st 
inner join score sc on sc.s_id = st.s_id and sc.c_id="01"
where st.s_id  in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id and sc2.c_id="02"
) and st.s_id in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id and sc2.c_id="03"
))
或
select * from student
where s_id not in 
	(select s_id from score group by s_id having count(c_id)=
		(select count(*) from course));
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 05   | 周梅   | 1991-12-01 | 女    |
| 06   | 吴兰   | 1992-03-01 | 女    |
| 07   | 郑竹   | 1989-07-01 | 女    |
| 08   | 王菊   | 1990-01-20 | 女    |
+------+--------+------------+-------+

# 12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息
select distinct st.* from student st 
left join score sc on sc.s_id=st.s_id
where sc.c_id in (
select sc2.c_id from student st2
left join score sc2 on sc2.s_id=st2.s_id
where st2.s_id ='01'
)
或
select distinct st.* 
from student st inner join score s on st.s_id = s.s_id 
where s.c_id = any (select c_id from score where s_id = '01');
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
| 05   | 周梅   | 1991-12-01 | 女    |
| 06   | 吴兰   | 1992-03-01 | 女    |
| 07   | 郑竹   | 1989-07-01 | 女    |
+------+--------+------------+-------+

# *13、查询和"01"号的同学学习的课程完全相同的其他同学的信息
select  st.* from student st 
left join score sc on sc.s_id=st.s_id
group by st.s_id
having group_concat(sc.c_id) = 
(
select  group_concat(sc2.c_id) from student st2
left join score sc2 on sc2.s_id=st2.s_id
where st2.s_id ='01'
)
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
+------+--------+------------+-------+

# 14、查询没学过"张三"老师讲授的任一门课程的学生姓名
select st.s_name from student st 
where st.s_id not in (
select sc.s_id from score sc 
inner join course c on c.c_id=sc.c_id
inner join teacher t on t.t_id=c.t_id and t.t_name="张三"
)
或
select s_name from student
where s_id not in 
	(select distinct st.s_id from student st left outer join score s on st.s_id = s.s_id where s.c_id in 
		(select c_id from course where t_id = 
			(select t_id from teacher where t_name = '张三')));
+--------+
| s_name |
+--------+
| 吴兰   |
| 王菊   |
+--------+

# 15、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
select st.s_id,st.s_name,avg(sc.s_score) from student st
left join score sc on sc.s_id=st.s_id
where sc.s_id in (
select sc.s_id from score sc 
where sc.s_score<60 or sc.s_score is NULL
group by sc.s_id having COUNT(sc.s_id)>=2
)
group by st.s_id
或
select st.s_id, st.s_name, avg(s.s_score) 
from student st left outer join score s on st.s_id = s.s_id
where st.s_id in (
select s_id from score 
where s_score < 60 or s_score is null
group by s_id having count(s_score) >= 2
)
group by st.s_id, st.s_name;
+------+--------+----------------+
| s_id | s_name | avg(s.s_score) |
+------+--------+----------------+
| 04   | 李云   |        33.3333 |
| 06   | 吴兰   |        32.5000 |
+------+--------+----------------+

# 16、检索"01"课程分数小于60，按分数降序排列的学生信息
select st.*,sc.s_score from student st 
inner join score sc on sc.s_id=st.s_id and sc.c_id="01" and sc.s_score<60
order by sc.s_score desc
或
select st.*, s.s_score
from student st inner join score s on st.s_id = s.s_id
where s.c_id = '01' and s.s_score < 60
order by s.s_score desc;
+------+--------+------------+-------+---------+
| s_id | s_name | s_birth    | s_sex | s_score |
+------+--------+------------+-------+---------+
| 04   | 李云   | 1990-08-06 | 男    |      50 |
| 06   | 吴兰   | 1992-03-01 | 女    |      31 |
+------+--------+------------+-------+---------+

# 17、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩，可加round,case when then else end 使显示更完美
select st.s_id,st.s_name,avg(sc4.s_score) "平均分",sc.s_score "语文",sc2.s_score "数学",sc3.s_score "英语" from student st
left join score sc  on sc.s_id=st.s_id  and sc.c_id="01"
left join score sc2 on sc2.s_id=st.s_id and sc2.c_id="02"
left join score sc3 on sc3.s_id=st.s_id and sc3.c_id="03"
left join score sc4 on sc4.s_id=st.s_id
group by st.s_id 
order by avg(sc4.s_score) desc
或
select st.s_id, st.s_name, avg(s.s_score) '平均成绩', s1.s_score '语文', s2.s_score '数学', s3.s_score '英语'
from student st 
left outer join score s on st.s_id = s.s_id 
left outer join score s1 on st.s_id = s1.s_id and s1.c_id = '01'
left outer join score s2 on st.s_id = s2.s_id and s2.c_id = '02'
left outer join score s3 on st.s_id = s3.s_id and s3.c_id = '03'
group by st.s_id, st.s_name
order by avg(s.s_score) desc;
+------+--------+----------+------+------+------+
| s_id | s_name | 平均成绩 | 语文 | 数学 | 英语 |
+------+--------+----------+------+------+------+
| 07   | 郑竹   |  93.5000 | NULL |   89 |   98 |
| 01   | 赵雷   |  89.6667 |   80 |   90 |   99 |
| 05   | 周梅   |  81.5000 |   76 |   87 | NULL |
| 03   | 孙风   |  80.0000 |   80 |   80 |   80 |
| 02   | 钱电   |  70.0000 |   70 |   60 |   80 |
| 04   | 李云   |  33.3333 |   50 |   30 |   20 |
| 06   | 吴兰   |  32.5000 |   31 | NULL |   34 |
| 08   | 王菊   |     NULL | NULL | NULL | NULL |
+------+--------+----------+------+------+------+

# *18.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
select c.c_id,c.c_name,max(sc.s_score) "最高分",MIN(sc2.s_score) "最低分",avg(sc3.s_score) "平均分" 
,((select count(s_id) from score where s_score>=60 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "及格率"
,((select count(s_id) from score where s_score>=70 and s_score<80 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "中等率"
,((select count(s_id) from score where s_score>=80 and s_score<90 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优良率"
,((select count(s_id) from score where s_score>=90 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优秀率"
from course c
left join score sc on sc.c_id=c.c_id 
left join score sc2 on sc2.c_id=c.c_id 
left join score sc3 on sc3.c_id=c.c_id 
group by c.c_id
或
select c.c_id, c.c_name, max(s.s_score) '最高分', min(s.s_score) '最低分', avg(s.s_score) '平均分',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id ) 
    from score where c_id = c.c_id and s_score >= 60) '及格率',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id ) 
    from score where c_id = c.c_id and (s_score between 70 and 79)) '中等率',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id )  
    from score where c_id = c.c_id and (s_score between 80 and 89)) '优良率',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id ) 
    from score where c_id = c.c_id and s_score >= 90) '优秀率' 
from course c left outer join score s on c.c_id = s.c_id
group by c.c_id, c.c_name;
+------+--------+--------+--------+---------+--------+--------+--------+--------+
| c_id | c_name | 最高分 | 最低分 | 平均分  | 及格率 | 中等率 | 优良率 | 优秀率 |
+------+--------+--------+--------+---------+--------+--------+--------+--------+
| 01   | 语文   |     80 |     31 | 64.5000 | 0.6667 | 0.3333 | 0.3333 | 0.0000 |
| 02   | 数学   |     90 |     30 | 72.6667 | 0.8333 | 0.0000 | 0.5000 | 0.1667 |
| 03   | 英语   |     99 |     20 | 68.5000 | 0.6667 | 0.0000 | 0.3333 | 0.3333 |
+------+--------+--------+--------+---------+--------+--------+--------+--------+

# *19、按各科成绩进行排序，并显示排名(实现不完全)
# mysql没有rank函数
# 加@score是为了防止用union all 后打乱了顺序
select c1.s_id,c1.c_id,c1.c_name,@score:=c1.s_score,@i:=@i+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="01" order by sc.s_score desc) c1 ,
(select @i:=0) a
union all 
select c2.s_id,c2.c_id,c2.c_name,c2.s_score,@ii:=@ii+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="02" order by sc.s_score desc) c2 ,
(select @ii:=0) aa 
union all
select c3.s_id,c3.c_id,c3.c_name,c3.s_score,@iii:=@iii+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="03" order by sc.s_score desc) c3;
set @iii=0;
或
select a.*
from (select st.s_id, st.s_name, c.c_name, s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '01' and s2.s_score >= s.s_score  and s2.c_id='01'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
union all
select b.*
from (select st.s_id, st.s_name, c.c_name, s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '02' and s2.s_score >= s.s_score  and s2.c_id='02'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) b
union all
select c.*
from (select st.s_id, st.s_name, c.c_name, s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '03' and s2.s_score >= s.s_score  and s2.c_id='03'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) c;
+------+--------+--------+---------+------+
| s_id | s_name | c_name | s_score | 排名 |
+------+--------+--------+---------+------+
| 01   | 赵雷   | 语文   |      80 |    1 |
| 03   | 孙风   | 语文   |      80 |    1 |
| 05   | 周梅   | 语文   |      76 |    2 |
| 02   | 钱电   | 语文   |      70 |    3 |
| 04   | 李云   | 语文   |      50 |    4 |
| 06   | 吴兰   | 语文   |      31 |    5 |
| 01   | 赵雷   | 数学   |      90 |    1 |
| 07   | 郑竹   | 数学   |      89 |    2 |
| 05   | 周梅   | 数学   |      87 |    3 |
| 03   | 孙风   | 数学   |      80 |    4 |
| 02   | 钱电   | 数学   |      60 |    5 |
| 04   | 李云   | 数学   |      30 |    6 |
| 01   | 赵雷   | 英语   |      99 |    1 |
| 07   | 郑竹   | 英语   |      98 |    2 |
| 02   | 钱电   | 英语   |      80 |    3 |
| 03   | 孙风   | 英语   |      80 |    3 |
| 06   | 吴兰   | 英语   |      34 |    4 |
| 04   | 李云   | 英语   |      20 |    5 |
+------+--------+--------+---------+------+

# 20、查询学生的总成绩并进行排名
select st.s_id,st.s_name
,(case when sum(sc.s_score) is null then 0 else sum(sc.s_score) end)
 from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id order by sum(sc.s_score) desc
或
select st.s_id, st.s_name, (case when sum(s.s_score) is null then 0 else sum(s.s_score) end) '总成绩'
from student st left outer join score s on st.s_id = s.s_id
group by st.s_id, st.s_name
order by sum(s.s_score) desc;
+------+--------+--------+
| s_id | s_name | 总成绩 |
+------+--------+--------+
| 01   | 赵雷   |    269 |
| 03   | 孙风   |    240 |
| 02   | 钱电   |    210 |
| 07   | 郑竹   |    187 |
| 05   | 周梅   |    163 |
| 04   | 李云   |    100 |
| 06   | 吴兰   |     65 |
| 08   | 王菊   |      0 |
+------+--------+--------+

# 21、查询不同老师所教不同课程平均分从高到低显示 
select t.t_id,t.t_name,c.c_name,avg(sc.s_score) from teacher t 
left join course c on c.t_id=t.t_id 
left join score sc on sc.c_id =c.c_id
group by t.t_id
order by avg(sc.s_score) desc
或
select t.t_id, t.t_name, c.c_name, avg(s.s_score) '平均分'
from teacher t left outer join course c on t.t_id = c.t_id
left outer join score s on c.c_id = s.c_id
group by t.t_id, t.t_name, c.c_name
order by avg(s.s_score) desc; 
+------+--------+--------+---------+
| t_id | t_name | c_name | 平均分  |
+------+--------+--------+---------+
| 01   | 张三   | 数学   | 72.6667 |
| 03   | 王五   | 英语   | 68.5000 |
| 02   | 李四   | 语文   | 64.5000 |
+------+--------+--------+---------+

# 22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
where s.c_id = '01' and (
s.s_score between
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '01' and s2.s_score >= s.s_score  and s2.c_id='01'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 3) 
and
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '01' and s2.s_score >= s.s_score  and s2.c_id='01'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 2)
)
union all
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
where s.c_id = '02' and (
s.s_score between
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '02' and s2.s_score >= s.s_score  and s2.c_id='02'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 3) 
and
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '02' and s2.s_score >= s.s_score  and s2.c_id='02'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 2)
)
union all
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
where s.c_id = '03' and (
s.s_score between
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '03' and s2.s_score >= s.s_score  and s2.c_id='03'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 3) 
and
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '03' and s2.s_score >= s.s_score  and s2.c_id='03'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 2)
);
+------+--------+------------+-------+--------+---------+
| s_id | s_name | s_birth    | s_sex | c_name | s_score |
+------+--------+------------+-------+--------+---------+
| 02   | 钱电   | 1990-12-21 | 男    | 语文   |      70 |
| 05   | 周梅   | 1991-12-01 | 女    | 语文   |      76 |
| 05   | 周梅   | 1991-12-01 | 女    | 数学   |      87 |
| 07   | 郑竹   | 1989-07-01 | 女    | 数学   |      89 |
| 02   | 钱电   | 1990-12-21 | 男    | 英语   |      80 |
| 03   | 孙风   | 1990-05-20 | 男    | 英语   |      80 |
| 07   | 郑竹   | 1989-07-01 | 女    | 英语   |      98 |
+------+--------+------------+-------+--------+---------+

# *23、统计各科成绩各分数段人数：课程编号,课程名称,[100-85],[85-70],[70-60],[60-0]及所占百分比
select c.c_id,c.c_name 
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=100 and sc.s_score>80)/(select count(1) from score sc where sc.c_id=c.c_id )) "100-85"
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=85 and sc.s_score>70)/(select count(1) from score sc where sc.c_id=c.c_id )) "85-70"
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=70 and sc.s_score>60)/(select count(1) from score sc where sc.c_id=c.c_id )) "70-60"
,((select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=60 and sc.s_score>=0)/(select count(1) from score sc where sc.c_id=c.c_id )) "60-0"
from course c order by c.c_id;
+------+--------+--------+--------+--------+--------+
| c_id | c_name | 100-85 | 85-70  | 70-60  | 60-0   |
+------+--------+--------+--------+--------+--------+
| 01   | 语文   | 0.0000 | 0.5000 | 0.1667 | 0.3333 |
| 02   | 数学   | 0.5000 | 0.1667 | 0.0000 | 0.3333 |
| 03   | 英语   | 0.3333 | 0.3333 | 0.0000 | 0.3333 |
+------+--------+--------+--------+--------+--------+
--count(1) 会统计表中的所有的记录数，包含字段为null 的记录。
--count(字段) 会统计该字段在表中出现的次数，忽略字段为null 的情况。即不统计字段为null 的记录。

# 24、查询学生平均成绩及其名次 
set @i=0; --推荐
select a.*,@i:=@i+1 from (
select st.s_id,st.s_name,round((case when avg(sc.s_score) is null then 0 else avg(sc.s_score) end),2) "平均分" from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id order by sc.s_score desc) a
或
select a.*, count(distinct b.平均成绩) '名次'
from
(select st.s_id, st.s_name, (case when avg(s.s_score) is null then 0 else avg(s.s_score) end) '平均成绩' 
from student st left join score s on st.s_id=s.s_id
group by st.s_id, st.s_name) a, 
(select st.s_id, (case when avg(s.s_score) is null then 0 else avg(s.s_score) end) '平均成绩' 
from student st left join score s on st.s_id=s.s_id
group by st.s_id) b
where a.平均成绩<=b.平均成绩
group by a.s_id
order by a.平均成绩 desc;
+------+--------+----------+------+
| s_id | s_name | 平均成绩 | 名次 |
+------+--------+----------+------+
| 07   | 郑竹   |  93.5000 |    1 |
| 01   | 赵雷   |  89.6667 |    2 |
| 05   | 周梅   |  81.5000 |    3 |
| 03   | 孙风   |  80.0000 |    4 |
| 02   | 钱电   |  70.0000 |    5 |
| 04   | 李云   |  33.3333 |    6 |
| 06   | 吴兰   |  32.5000 |    7 |
| 08   | 王菊   |   0.0000 |    8 |
+------+--------+----------+------+

# 25、查询各科成绩前三名的记录
select a.*
from (
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_id='01' and s.s_score between (
select distinct a1.s_score 
from (
select a2.s_id, a2.s_score, count(distinct a3.s_score) '名次'
from score a2, score a3
where a2.c_id='01' and a3.c_id='01' and a2.s_score<=a3.s_score
group by a2.s_id, a2.s_score
) a1
where a1.名次=3)
and
(select max(s_score) from score where c_id='01')
) a
union all	
select b.*
from (
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_id='02' and s.s_score between (
select distinct a1.s_score 
from (
select a2.s_id, a2.s_score, count(distinct a3.s_score) '名次'
from score a2, score a3
where a2.c_id='02' and a3.c_id='02' and a2.s_score<=a3.s_score
group by a2.s_id, a2.s_score
) a1
where a1.名次=3)
and
(select max(s_score) from score where c_id='02')
) b
union all	
select c.*
from (
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_id='03' and s.s_score between (
select distinct a1.s_score 
from (
select a2.s_id, a2.s_score, count(distinct a3.s_score) '名次'
from score a2, score a3
where a2.c_id='03' and a3.c_id='03' and a2.s_score<=a3.s_score
group by a2.s_id, a2.s_score
) a1
where a1.名次=3)
and
(select max(s_score) from score where c_id='03')
) c;
+------+--------+------------+-------+--------+---------+
| s_id | s_name | s_birth    | s_sex | c_name | s_score |
+------+--------+------------+-------+--------+---------+
| 01   | 赵雷   | 1990-01-01 | 男    | 语文   |      80 |
| 02   | 钱电   | 1990-12-21 | 男    | 语文   |      70 |
| 03   | 孙风   | 1990-05-20 | 男    | 语文   |      80 |
| 05   | 周梅   | 1991-12-01 | 女    | 语文   |      76 |
| 01   | 赵雷   | 1990-01-01 | 男    | 数学   |      90 |
| 05   | 周梅   | 1991-12-01 | 女    | 数学   |      87 |
| 07   | 郑竹   | 1989-07-01 | 女    | 数学   |      89 |
| 01   | 赵雷   | 1990-01-01 | 男    | 英语   |      99 |
| 02   | 钱电   | 1990-12-21 | 男    | 英语   |      80 |
| 03   | 孙风   | 1990-05-20 | 男    | 英语   |      80 |
| 07   | 郑竹   | 1989-07-01 | 女    | 英语   |      98 |
+------+--------+------------+-------+--------+---------+

# 26、查询每门课程被选修的学生数 
select c.c_id,c.c_name,count(1) from course c 
left join score sc on sc.c_id=c.c_id
inner join student st on st.s_id=c.c_id --？？？
group by st.s_id
或
select c.c_id, c.c_name, count(s.s_id) '选修人数'
from course c
left join score s on c.c_id=s.c_id
group by c.c_id, c.c_name;
+------+--------+----------+
| c_id | c_name | 选修人数 |
+------+--------+----------+
| 01   | 语文   |        6 |
| 02   | 数学   |        6 |
| 03   | 英语   |        6 |
+------+--------+----------+

# 27、查询出只有两门课程的全部学生的学号和姓名
select st.s_id,st.s_name from student st 
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id 
group by st.s_id having count(1)=2
或
select st.s_id, st.s_name
from student st
left join score s on st.s_id=s.s_id
group by st.s_id, st.s_name
having count(s.c_id)=2;
+------+--------+
| s_id | s_name |
+------+--------+
| 05   | 周梅   |
| 06   | 吴兰   |
| 07   | 郑竹   |
+------+--------+

# 28、查询男生、女生人数
select st.s_sex,count(1) from student st group by st.s_sex
+-------+----------+
| s_sex | count(1) |
+-------+----------+
| 男    |        4 |
| 女    |        4 |
+-------+----------+

# 29、查询名字中含有"风"字的学生信息
select st.* from student st where st.s_name like "%风%";
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 03   | 孙风   | 1990-05-20 | 男    |
+------+--------+------------+-------+

# 30、查询同名同性学生名单，并统计同名人数 
select st.*,count(1) from student st group by st.s_name,st.s_sex having count(1)>1
Empty set (0.00 sec)

# 31、查询1990年出生的学生名单
select st.* from student st where st.s_birth like "1990%";
或
select * from student where year(s_birth) = '1990';
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
| 08   | 王菊   | 1990-01-20 | 女    |
+------+--------+------------+-------+

# 32、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 
select c.c_id,c.c_name,avg(sc.s_score) from course c
inner join score sc on sc.c_id=c.c_id  
group by c.c_id, c.c_name order by avg(sc.s_score) desc,c.c_id asc;
+------+--------+-----------------+
| c_id | c_name | avg(sc.s_score) |
+------+--------+-----------------+
| 02   | 数学   |         72.6667 |
| 03   | 英语   |         68.5000 |
| 01   | 语文   |         64.5000 |
+------+--------+-----------------+

# 33、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
select st.s_id,st.s_name,avg(sc.s_score) from student st
left join score sc on sc.s_id=st.s_id
group by st.s_id having avg(sc.s_score)>=85;
+------+--------+-----------------+
| s_id | s_name | avg(sc.s_score) |
+------+--------+-----------------+
| 01   | 赵雷   |         89.6667 |
| 07   | 郑竹   |         93.5000 |
+------+--------+-----------------+

# 34、查询课程名称为"数学"，且分数低于60的学生姓名和分数 
select st.s_name,sc.s_score from student st
inner join score sc on sc.s_id=st.s_id and sc.s_score<60
inner join course c on c.c_id=sc.c_id and c.c_name ="数学" 
或
select st.s_name, s.s_score
from student st
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_name='数学' and s.s_score<60;
+--------+---------+
| s_name | s_score |
+--------+---------+
| 李云   |      30 |
+--------+---------+

# 35、查询所有学生的课程及分数情况；
select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
left join course c on c.c_id =sc.c_id
order by st.s_id,c.c_name;
+------+--------+--------+---------+
| s_id | s_name | c_name | s_score |
+------+--------+--------+---------+
| 01   | 赵雷   | 数学   |      90 |
| 01   | 赵雷   | 英语   |      99 |
| 01   | 赵雷   | 语文   |      80 |
| 02   | 钱电   | 数学   |      60 |
| 02   | 钱电   | 英语   |      80 |
| 02   | 钱电   | 语文   |      70 |
| 03   | 孙风   | 数学   |      80 |
| 03   | 孙风   | 英语   |      80 |
| 03   | 孙风   | 语文   |      80 |
| 04   | 李云   | 数学   |      30 |
| 04   | 李云   | 英语   |      20 |
| 04   | 李云   | 语文   |      50 |
| 05   | 周梅   | 数学   |      87 |
| 05   | 周梅   | 语文   |      76 |
| 06   | 吴兰   | 英语   |      34 |
| 06   | 吴兰   | 语文   |      31 |
| 07   | 郑竹   | 数学   |      89 |
| 07   | 郑竹   | 英语   |      98 |
| 08   | 王菊   | NULL   |    NULL |
+------+--------+--------+---------+

# 36、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
select st2.s_id,st2.s_name,c2.c_name,sc2.s_score from student st2
left join score sc2 on sc2.s_id=st2.s_id
left join course c2 on c2.c_id=sc2.c_id 
where st2.s_id in(
select st.s_id from student st 
left join score sc on sc.s_id=st.s_id 
group by st.s_id having min(sc.s_score)>70)
order by s_id;
+------+--------+--------+---------+
| s_id | s_name | c_name | s_score |
+------+--------+--------+---------+
| 01   | 赵雷   | 语文   |      80 |
| 01   | 赵雷   | 数学   |      90 |
| 01   | 赵雷   | 英语   |      99 |
| 03   | 孙风   | 语文   |      80 |
| 03   | 孙风   | 数学   |      80 |
| 03   | 孙风   | 英语   |      80 |
| 05   | 周梅   | 语文   |      76 |
| 05   | 周梅   | 数学   |      87 |
| 07   | 郑竹   | 数学   |      89 |
| 07   | 郑竹   | 英语   |      98 |
+------+--------+--------+---------+

# 37、查询不及格的课程
select st.s_id,c.c_name,st.s_name,sc.s_score from student st
inner join score sc on sc.s_id=st.s_id and  sc.s_score<60
inner join course c on c.c_id=sc.c_id 
或
select st.s_id, st.s_name, c.c_name, s.s_score from student st
left join score s on s.s_id=st.s_id
left join course c on c.c_id=s.c_id
where s.s_score<60;
+------+--------+--------+---------+
| s_id | c_name | s_name | s_score |
+------+--------+--------+---------+
| 04   | 语文   | 李云   |      50 |
| 04   | 数学   | 李云   |      30 |
| 04   | 英语   | 李云   |      20 |
| 06   | 语文   | 吴兰   |      31 |
| 06   | 英语   | 吴兰   |      34 |
+------+--------+--------+---------+

# 38、查询课程编号为01且课程成绩在80分以上的学生的学号和姓名
select st.s_id,st.s_name,sc.s_score from student st
inner join score sc on sc.s_id=st.s_id and sc.c_id="01" and sc.s_score>=80;
+------+--------+---------+
| s_id | s_name | s_score |
+------+--------+---------+
| 01   | 赵雷   |      80 |
| 03   | 孙风   |      80 |
+------+--------+---------+

# 39、求每门课程的学生人数
select c.c_id,c.c_name,count(1) from course c
inner join score sc on sc.c_id=c.c_id
group by c.c_id
或
select c.c_id, c.c_name, count(st.s_id)
from course c 
left join score s on c.c_id=s.c_id
left join student st on s.s_id=st.s_id
group by c.c_id, c.c_name;
+------+--------+----------------+
| c_id | c_name | count(st.s_id) |
+------+--------+----------------+
| 01   | 语文   |              6 |
| 02   | 数学   |              6 |
| 03   | 英语   |              6 |
+------+--------+----------------+

# 40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩 
select st.*,c.c_name,sc.s_score,t.t_name from student st
inner join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id 
inner join teacher t on t.t_id=c.t_id and  t.t_name="张三"
order by sc.s_score desc
limit 0,1
或
select st.*, s.s_score, t.t_name, c.c_name
from student st
left join score s on st.s_id=s.s_id
left join course c on s.c_id=c.c_id
left join teacher t on c.t_id=t.t_id
where t.t_name='张三'
and s.s_score = (
select distinct max(s2.s_score)
from score s2 left join course c2 on s2.c_id=c2.c_id
left join teacher t2 on c2.t_id=t2.t_id
where t2.t_name='张三'
);
+------+--------+------------+-------+---------+--------+--------+
| s_id | s_name | s_birth    | s_sex | s_score | t_name | c_name |
+------+--------+------------+-------+---------+--------+--------+
| 01   | 赵雷   | 1990-01-01 | 男    |      90 | 张三   | 数学   |
+------+--------+------------+-------+---------+--------+--------+

# *41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩 
select st.s_id,st.s_name,sc.c_id,sc.s_score,c.c_name from student st 
left join score sc on sc.s_id=st.s_id
left join course c on c.c_id=sc.c_id
where (
select count(1) from student st2 
left join score sc2 on sc2.s_id=st2.s_id
left join course c2 on c2.c_id=sc2.c_id
where sc.s_score=sc2.s_score and c.c_id!=c2.c_id 
)>1

# 42、查询每门功成绩最好的前两名 
select a.* from (select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
order by sc.s_score desc limit 0,2) a
union all
select b.* from (select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="02"
order by sc.s_score desc limit 0,2) b
union all
select c.* from (select st.s_id,st.s_name,c.c_name,sc.s_score from student st
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="03"
order by sc.s_score desc limit 0,2) c;
+------+--------+--------+---------+
| s_id | s_name | c_name | s_score |
+------+--------+--------+---------+
| 01   | 赵雷   | 语文   |      80 |
| 03   | 孙风   | 语文   |      80 |
| 01   | 赵雷   | 数学   |      90 |
| 07   | 郑竹   | 数学   |      89 |
| 01   | 赵雷   | 英语   |      99 |
| 07   | 郑竹   | 英语   |      98 |
+------+--------+--------+---------+
-- 借鉴(更准确,漂亮):
select a.s_id,a.c_id,a.s_score from score a
where (select COUNT(1) from score b where b.c_id=a.c_id and b.s_score>=a.s_score)<=2 order by a.c_id
  

# 43、统计每门课程的学生选修人数（超过5人的课程才统计）。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 sc.c_id,count(1) from score sc
left join course c on c.c_id=sc.c_id
group by c.c_id having count(1)>5
order by count(1) desc,sc.c_id asc
或
select c.c_id, count(s.s_id)
from course c left join score s on c.c_id=s.c_id
group by c.c_id
having count(s.s_id)>5
order by count(s.s_id) desc, c.c_id;
+------+---------------+
| c_id | count(s.s_id) |
+------+---------------+
| 01   |             6 |
| 02   |             6 |
| 03   |             6 |
+------+---------------+

# 44、检索至少选修两门课程的学生学号 
select st.s_id from student st 
left join score sc on sc.s_id=st.s_id
group by st.s_id having count(1)>=2;
+------+
| s_id |
+------+
| 01   |
| 02   |
| 03   |
| 04   |
| 05   |
| 06   |
| 07   |
+------+

# 45、查询选修了全部课程的学生信息
select st.*
from student st left join score s on st.s_id=s.s_id
group by st.s_id
having count(s.c_id)=(select count(*) from course)
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
+------+--------+------------+-------+

# *46、查询各学生的年龄(实岁)
select st.*,timestampdiff(year,st.s_birth,now()) from student st

# 47、查询本周过生日的学生
select st.* from student st 
where week(now())=week(date_format(st.s_birth,'%Y%m%d'))

# 48、查询下周过生日的学生(还要排除闰年情况)
select st.* from student st 
where week(now())+1=week(date_format(st.s_birth,'%Y%m%d'))

# 49、查询本月过生日的学生
select st.* from student st 
where month(now())=month(date_format(st.s_birth,'%Y%m%d'))

# *50、查询下月过生日的学生
# 注意:当 当前月为12时,用month(now())+1为13而不是1,可用timestampadd()函数或mod取模
select st.* from student st 
where month(timestampadd(month,1,now()))=month(date_format(st.s_birth,'%Y%m%d'))
-- 或
select st.* from student st where (month(now()) + 1) mod 12 = month(date_format(st.s_birth,'%Y%m%d'))