mysql45 练习答案

记录一下自己对题目的理解思路还有做法
-- mysql45
练习数据
数据表
--1.学生表 Student(SId,Sname,Sage,Ssex)
SId 学生编号,Sname 学生姓名,Sage 出生年月,Ssex 学生性别
--2.课程表 Course(CId,Cname,TId)
CId 课程编号,Cname 课程名称,TId 教师编号
--3.教师表 Teacher(TId,Tname)
TId 教师编号,Tname 教师姓名
--4.成绩表 SC(SId,CId,score)
SId 学生编号,CId 课程编号,score 分数

创建测试数据
学生表 Student：
导入数据方法：将以下 mysql 语句，完整复制到 workbench 语句窗口（或者是 mysql 的黑窗口），然后运行即可导入，不需要另外创建表，下面表的操作一样。这些语句第一条是创建表（create table），后面都是插入数据到表中（insert into table ）。
create table Student(SId varchar(10),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('09' , '张三' , '2017-12-20' , '女');
insert into Student values('10' , '李四' , '2017-12-25' , '女');
insert into Student values('11' , '李四' , '2017-12-30' , '女');
insert into Student values('12' , '赵六' , '2017-01-01' , '女');
insert into Student values('13' , '孙七' , '2018-01-01' , '女');

科目表 Course
create table Course(CId varchar(10),Cname nvarchar(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');

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);

-- 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 *
FROM
sc a LEFT JOIN student stu
ON a.sid = stu.sid
INNER JOIN sc b
ON a.sid = b.sid AND a.cid = 01 AND b.cid = 02
WHERE a.score > b.score;

-- 1.1 查询同时存在" 01 "课程和" 02 "课程的情况
SELECT *
FROM sc a INNER JOIN sc b
ON a.SId = b.SId AND a.CId = 01 AND b.CId = 02;

-- 1.2查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null )
SELECT *
FROM sc a
LEFT JOIN sc b
ON a.SId = b.SId and b.CId = 02
WHERE a.CId = 01;

-- 1.3 查询不存在" 01 "课程但存在" 02 "课程的情况
SELECT *
FROM sc a
LEFT JOIN sc b
ON a.SId = b.SId and b.CId = 01
WHERE a.CId = 02 AND b.CId IS NULL;

-- 2.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
-- 问题：为什么这里的group by 的having 条件查询，可以直接使用 s_avg 别名？
SELECT stu.SId,stu.Sname,avg(a.score) s_avg
FROM student stu INNER JOIN sc a
ON stu.SId = a.SId
GROUP BY stu.SId
HAVING s_avg >= 60;
-- 答：因为sql语句执行顺序：先执行select，再执行having

-- 3.查询在 SC 表存在成绩的学生信息
SELECT DISTINCT stu.*
FROM sc a LEFT JOIN student stu
on a.SId = stu.SId;

-- 4.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)
SELECT a.sid,a.sname,COUNT(b.CId),SUM(b.score)
FROM student a LEFT JOIN sc b
ON a.sid = b.SId
GROUP BY a.sid;

-- 4.1 查有成绩的学生信息
SELECT a.sid,a.sname,COUNT(b.CId),SUM(b.score)
FROM student a INNER JOIN sc b
ON a.sid = b.SId
GROUP BY a.sid;

-- 5.查询「李」姓老师的数量
SELECT COUNT(1)
FROM teacher
WHERE tname LIKE '李%';

-- 6.查询学过「张三」老师授课的同学的信息
-- a、查询张三老师授课的课程编号
SELECT cid
FROM course
WHERE tid = (
SELECT tid FROM teacher WHERE tname = '张三');
-- b、查询学过「张三」老师授课的同学的信息
SELECT b.*
FROM sc a INNER JOIN student b
ON a.SId = b.SId
WHERE a.CId = (SELECT cid FROM course
WHERE tid = (SELECT tid FROM teacher WHERE tname = '张三'));

-- 7.查询没有学全所有课程的同学的信息
SELECT a.*
FROM student a LEFT JOIN sc b
ON a.SId = b.SId
GROUP BY a.SId
HAVING COUNT(b.CId) < (select count(cid) from course);

-- 8.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
-- a、查询01同学学的课程id
SELECT cid
FROM sc
WHERE sid = 01
-- b、查询学过01同学课程的同学注意去重
SELECT DISTINCT a.*
FROM student a LEFT JOIN sc b
ON a.sid = b.sid
WHERE b.cid IN (SELECT cid FROM sc WHERE sid = 01);

-- 9.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
-- 找到与01同学课程不同的同学
SELECT DISTINCT sid
FROM sc
where cid NOT IN (SELECT cidFROM sc WHERE sid = 01);

-- 排除以上同学，并且限制课程总数
SELECT a.*
FROM student a LEFT JOIN sc b
ON a.SId = b.SId
WHERE a.SId NOT IN (SELECT DISTINCT sid FROM sc WHERE cid NOT IN
(SELECT cid FROM sc WHERE sid = 01))
AND a.SId != 01
GROUP BY a.sid
HAVING COUNT(b.CId) = (SELECT COUNT(cid) FROM sc WHERE sid = 01);

-- 10.查询没学过"张三"老师讲授的任一门课程的学生姓名
-- 查询张三老师的授课id
SELECT cid
FROM course
WHERE tid = (
SELECT tid FROM teacher WHERE tname = '张三');
-- 查询上过张三老师课程的学生id
SELECT sid
FROM cs
WHERE cid IN (SELECT cid
FROM course
WHERE tid = (
SELECT tid FROM teacher WHERE tname = '张三'));

-- 排除以上学生
SELECT sname
FROM student
WHERE sid NOT IN (SELECT sid
FROM sc
WHERE cid IN (SELECT cid
FROM course
WHERE tid = (
SELECT tid FROM teacher WHERE tname = '张三')));

-- 11.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
SELECT a.sid,a.Sname,avg(b.score)
FROM student a LEFT JOIN sc b
ON a.SId = b.SId
WHERE b.score < 60
GROUP BY sid
HAVING COUNT(1) >= 2;

-- 12、检索" 01 "课程分数小于 60，按分数降序排列的学生信息
SELECT b.*
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
WHERE a.CId = 01
AND a.score < 60
ORDER BY a.score DESC;

-- 13、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
-- 平均成绩
SELECT SId,avg(score) avg_stu
FROM sc
GROUP BY SId;
-- 表连接
SELECT a.SId,b.cid,b.score,avg_stu
FROM student a LEFT JOIN sc b
ON a.SId = b.SId
LEFT JOIN (SELECT SId,avg(score) avg_stu FROM sc GROUP BY SId) c
ON a.SId = c.SId
GROUP BY a.SId,b.cid
ORDER BY avg_stu DESC;
-- 要关联上student表，题目要求所有学生

-- 14、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
a.CId 课程ID,
b.Cname 课程name,
COUNT(a.SId) 选修人数,
MAX(a.score) 最高分,
MIN(a.score) 最低分,
AVG(a.score) 平均分,
SUM(IF(a.score >= 60,1,0))/COUNT(a.SId) 及格率,
SUM(IF(a.score >= 70 AND a.score < 80,1,0))/COUNT(a.SId) 中等率,
SUM(IF(a.score >= 80 AND a.score <= 90,1,0))/COUNT(a.SId) 优良率,
SUM(IF(a.score >= 90,1,0))/COUNT(a.SId) 优秀率
FROM sc a LEFT JOIN course b
ON a.CId = b.CId
GROUP BY a.CId
ORDER BY COUNT(a.SId) DESC,a.CId ;

-- 15、按各科成绩进行排序，并显示排名， Score 重复时也继续排名
-- 对题目的理解：先按学科分组分数相同时，排名相同，下一个分数的排名为前面的总人数+1
SELECT a.CId,a.SId,a.score,COUNT(b.score)+1 rk
FROM sc a LEFT JOIN sc b
ON a.CId = b.CId AND a.score < b.score
GROUP BY a.CId,a.SId
ORDER BY a.CId,a.score DESC;

-- 15.1 按各科成绩进行排序，并显示排名， Score 重复时合并名次
-- 对题目的理解：分数相同时，排名相同，下一个分数的排名为上一人排名+1
SELECT a.CId,a.SId,a.score,COUNT(DISTINCT b.score)+1 rk
FROM sc a LEFT JOIN sc b
ON a.CId = b.CId AND a.score < b.score
GROUP BY a.CId,a.SId
ORDER BY a.CId,a.score DESC;

-- 16、查询学生的总成绩，并进行排名，总分重复时保留名次空缺
-- 查询总成绩
SELECT SId,SUM(score) scores
FROM sc
GROUP BY SId;
-- 利用count进行排名
SELECT a.SId,a.scores,COUNT(b.scores)+1
FROM
(SELECT SId,SUM(score) scores
FROM sc
GROUP BY SId
) a LEFT JOIN
(SELECT SId,SUM(score) scores
FROM sc
GROUP BY SId
) b
ON a.scores < b.scores
GROUP BY a.SId
ORDER BY a.scores DESC;
-- 16.1 查询学生的总成绩，并进行排名，总分重复时不保留名次空缺
SELECT a.SId,a.scores,COUNT(DISTINCT b.scores)+1
FROM
(SELECT SId,SUM(score) scores
FROM sc
GROUP BY SId
) a LEFT JOIN
(SELECT SId,SUM(score) scores
FROM sc
GROUP BY SId
) b
ON a.scores < b.scores
GROUP BY a.SId
ORDER BY a.scores DESC;

-- 17.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
SELECT
a.cid,
b.cname,
SUM(IF(a.score >= 85,1,0))/COUNT(1) '100-85',
SUM(IF(a.score >= 70 AND a.score < 85,1,0))/COUNT(1) '85-70',
SUM(IF(a.score >= 60 AND a.score < 70,1,0))/COUNT(1) '70-60',
SUM(IF(a.score >= 0 AND a.score < 60,1,0))/COUNT(1) '60-0'
FROM sc a LEFT JOIN course b
ON a.CId = b.CId
GROUP BY cid;

-- 18.查询各科成绩前三名的记录
-- 思路：前三名转化为若大于此成绩的数量少于3即为前三名。
SELECT a.CId,a.SId,a.score
FROM sc a
WHERE
(SELECT COUNT(1) FROM sc b WHERE b.score > a.score AND a.CId = b.CId) < 3
ORDER BY a.CId,a.score DESC;

-- 19.查询每门课程被选修的学生数
SELECT CId,COUNT(1)
FROM sc
GROUP BY CId;

-- 20.查询出只选修两门课程的学生学号和姓名
SELECT a.SId,b.Sname
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
GROUP BY a.SId
HAVING COUNT(a.CId) = 2;

-- 21.查询男生、女生人数
SELECT Ssex,COUNT(1)
FROM student
GROUP BY Ssex;

-- 22、查询名字中含有「风」字的学生信息
SELECT *
FROM student
WHERE Sname LIKE '%风%';

-- 23.查询同名同性学生名单，并统计同名同姓人数
SELECT Sname,COUNT(1)
FROM student
GROUP BY Sname
HAVING COUNT(1) > 1;

-- 22.查询 1990 年出生的学生名单

SELECT * FROM student WHERE YEAR(Sage) = 1990;

-- 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
SELECT CId,AVG(score) avg_sco
FROM sc
GROUP BY CId
ORDER BY avg_sco DESC,CId;

-- 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
SELECT a.SId,b.Sname,AVG(a.score)
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
GROUP BY a.SId
HAVING AVG(a.score) >= 85;

-- 27、查询课程名称为「数学」，且分数低于 60 的学生姓名和分数

SELECT b.Sname,a.score
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
WHERE a.CId = (SELECT CId FROM course WHERE Cname = '数学')
AND a.score < 60;

-- 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）
SELECT *
FROM student a LEFT JOIN sc b
ON a.SId = b.SId;

-- 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
SELECT b.Sname,a.CId,a.score
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
WHERE a.score > 70;

-- 30.查询存在不及格的课程
SELECT DISTINCT a.CId,b.Cname
FROM sc a LEFT JOIN course b
ON a.CId = b.CId
WHERE a.score < 60;

-- 31.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 a.SId,b.Sname
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
WHERE a.CId = 01
AND a.score > 80;

-- 32.求每门课程的学生人数
SELECT CId,COUNT(1)
FROM sc
GROUP BY CId;

-- 33、假设成绩不重复，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
-- 张三老师课程id
-- 查询上过张三老师课程的学生id
SELECT *
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
WHERE a.cid IN
(SELECT cid
FROM course
WHERE tid = (SELECT tid FROM teacher WHERE tname = '张三')
)
ORDER BY score DESC
LIMIT 1;

-- 34、假设成绩有重复的情况下，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

-- 找到最高分
SELECT MAX(score)
FROM sc
WHERE cid IN
(SELECT cid
FROM course
WHERE tid = (SELECT tid FROM teacher WHERE tname = '张三')
);
-- 找成绩为最高分的同学
SELECT *
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
WHERE a.cid IN
(SELECT cid FROM course
WHERE tid = (SELECT tid FROM teacher WHERE tname = '张三')
)
AND
a.score = (SELECT MAX(score) FROM sc WHERE cid IN
(SELECT cid FROM course WHERE tid =
(SELECT tid FROM teacher WHERE tname = '张三'))
);

-- 35、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
-- cha'de'shi
SELECT a.SId,a.CId,a.score,b.SId,b.CId
FROM sc a LEFT JOIN sc b
ON a.score = b.score
WHERE a.CId != b.CId;

SELECT a.SId,a.CId,a.score
FROM sc a LEFT JOIN sc b
ON a.score = b.score
WHERE a.CId != b.CId
GROUP BY a.SId,a.CId;

-- 36.查询每门功成绩最好的前两名
SELECT a.CId,a.SId,a.score
FROM sc a
WHERE (SELECT COUNT(1) FROM sc b WHERE a.CId = b.CId AND a.score < b.score) < 2
ORDER BY a.CId,a.score DESC;

-- 37.统计每门课程的学生选修人数（超过 5 人的课程才统计）
SELECT CId,COUNT(1)
FROM sc
GROUP BY CId
HAVING COUNT(1)>5;

-- 38.检索至少选修两门课程的学生学号
SELECT DISTINCT sid
FROM sc
GROUP BY SId
HAVING COUNT(1) >= 2;

-- 39、查询选修了全部课程的学生信息
SELECT b.*
FROM sc a LEFT JOIN student b
ON a.SId = b.SId
GROUP BY a.SId
HAVING COUNT(1) = (SELECT COUNT(1) FROM course);

-- 40.查询各学生的年龄，只按年份来算
SELECT
sid,
YEAR(NOW()) - YEAR(Sage) age
FROM student;

-- 41、按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一
SELECT
sid,
TIMESTAMPDIFF(YEAR,Sage,NOW()) age
FROM student;

-- 42.查询本周过生日的学生
SELECT *
FROM student
WHERE WEEKOFYEAR(Sage) = WEEKOFYEAR(NOW());

-- 43、查询下周过生日的学生
SELECT *
FROM student
WHERE WEEKOFYEAR(Sage) = WEEKOFYEAR(NOW())+1;

-- 44.查询本月过生日的学生
SELECT *
FROM student
WHERE MONTH(Sage) = MONTH(NOW());

-- 45.查询下月过生日的学生
SELECT *
FROM student
WHERE MONTH(Sage) = MONTH(DATE_ADD(NOW(),INTERVAL 1 MONTH));