一、环境准备
sql脚本如下：
create table Student(sid varchar(10),sname varchar(10),sage datetime,ssex nvarchar(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 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'); 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);
脚本说明：
一共4张表，分别对应学生信息（Student）、课程信息（Course）、教师信息（Teacher）以及成绩信息（SC）
二、正文部分
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 student.*,t3.sid FROM (SELECT t1.sid,t1.score FROM (SELECT sid,score FROM sc WHERE cid = "01") as t1 JOIN (SELECT sid,score FROM sc WHERE cid = "02") as t2 ON t1.sid = t2.sid WHERE t1.score > t2.score) as t3 JOIN student ON t3.sid = student.sid;
结果：
+-----+-------+---------------------+------+-----+ | sid | sname | sage | ssex | sid | +-----+-------+---------------------+------+-----+ | 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 02 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | 04 | +-----+-------+---------------------+------+-----+ 2 rows in set
解析：
先将课程为01和02的课程及对应分数筛选出来，再join，on为01.sid = 02.sid，条件为01.score >02.score，结果'存'为新表t3，再将Student表和t3表join
2、查询学生选课存在" 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 (SELECT * FROM sc WHERE cid = "01") as t1 LEFT JOIN (SELECT * FROM sc WHERE cid = "02") as t2 ON t1.sid = t2.sid;
结果：
+-----+-----+-------+------+------+-------+ | sid | cid | score | sid | cid | score | +-----+-----+-------+------+------+-------+ | 01 | 01 | 80.0 | 01 | 02 | 90.0 | | 02 | 01 | 70.0 | 02 | 02 | 60.0 | | 03 | 01 | 80.0 | 03 | 02 | 80.0 | | 04 | 01 | 50.0 | 04 | 02 | 30.0 | | 05 | 01 | 76.0 | 05 | 02 | 87.0 | | 06 | 01 | 31.0 | NULL | NULL | NULL | +-----+-----+-------+------+------+-------+ 6 rows in set
解析：
即找出学生选了01课程没有选02课程的情况，用left join即可
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
#多表联合查询 SELECT sc.sid,student.sname,avg(sc.score) FROM sc ,student WHERE sc.sid = student.sid GROUP BY sc.sid HAVING avg(sc.score) > 60; #多表连接查询 SELECT sc.sid,student.sname,avg(sc.score) FROM sc JOIN student on sc.sid = student.sid GROUP BY sc.sid HAVING avg(sc.score) > 60;
结果：
+-----+-------+---------------+ | sid | sname | avg(sc.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 | +-----+-------+---------------+ 5 rows in set
解析：
首先确定的是两张表，student和sc，这里使用多表联合查询和多表连接查的方式都可以，关联条件是sid，然后分组，最后加一个having函数，条件是平均成绩大于60，即可查询出来
4、查询在 SC 表存在成绩的学生信息
#多表联合查询方式 SELECT t1.*,t2.score FROM student t1, sc t2 WHERE t1.sid = t2.sid GROUP BY t1.sid; #多表连接查询方式 SELECT a.*,b.score FROM student as a JOIN sc AS b ON a.sid = b.sid GROUP BY a.sid;
结果：
+-----+-------+---------------------+------+-------+ | sid | sname | sage | ssex | score | +-----+-------+---------------------+------+-------+ | 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 80.0 | | 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 70.0 | | 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 80.0 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | 50.0 | | 05 | 周梅 | 1991-12-01 00:00:00 | 女 | 76.0 | | 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | 31.0 | | 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 89.0 | +-----+-------+---------------------+------+-------+ 7 rows in set
解析：
确定是两个表,student和sc，关联条件还是sid消除笛卡尔积，然后再group by，最后select 取需要的信息
5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的成绩总和
#多表联合查询方式 SELECT t1.sid as 学生编号,t1.sname as 学生姓名,COUNT(t2.cid) as 选课总数,SUM(t2.score) as 课程成绩总和 FROM student t1, sc t2 WHERE t1.sid = t2.sid GROUP BY t1.sid; #多表连接查询 SELECT t1.sid as 学生编号,t1.sname as 学生姓名,COUNT(t2.cid) as 选课总数,SUM(t2.score) as 课程成绩总和 FROM student t1 JOIN sc t2 ON t1.sid = t2.sid GROUP BY t1.sid;
结果：
+----------+----------+----------+--------------+ | 学生编号 | 学生姓名 | 选课总数 | 课程成绩总和 | +----------+----------+----------+--------------+ | 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 | +----------+----------+----------+--------------+ 7 rows in set
解析：
两个聚合函数（统计函数）一个count(cid),一个sum(score),同样join student表和sc表，再group by sid即可
6、查询「李」姓老师的数量
SELECT COUNT(t.tid) FROM teacher t WHERE t.tname like "%李%";
结果：
+--------------+ | COUNT(t.tid) | +--------------+ | 1 | +--------------+ 1 row in set
解析：
count加条件函数加通配符即可
7、查询学过「张三」老师授课的同学的信息
SELECT f.*,e.tname FROM (SELECT d.sid,c.tname FROM (SELECT a.tname,b.cid FROM teacher AS a JOIN course AS b ON a.tid = b.tid WHERE a.tname = '张三') AS c JOIN sc AS d ON c.cid = d.cid) AS e JOIN student AS f ON e.sid = f.sid
结果：
+-----+-------+---------------------+------+-------+ | sid | sname | sage | ssex | tname | +-----+-------+---------------------+------+-------+ | 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 张三 | | 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 张三 | | 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 张三 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | 张三 | | 05 | 周梅 | 1991-12-01 00:00:00 | 女 | 张三 | | 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 张三 | +-----+-------+---------------------+------+-------+ 6 rows in set
解析：
四表连接，teacher表里的tid与course表里的tid，条件为tname=‘张三’，再course表里的cid与sc表里的cid，最后sc表里的sid与student里的sid
8、查询没有学全所有课程的同学的信息
SELECT a.*,count(b.cid) AS 所学课程数 FROM student AS a LEFT JOIN sc AS b ON a.sid = b.sid GROUP BY b.sid HAVING COUNT(b.cid)< (SELECT COUNT(c.cid) FROM course as c);
结果：
+-----+-------+---------------------+------+------------+ | sid | sname | sage | ssex | 所学课程数 | +-----+-------+---------------------+------+------------+ | 05 | 周梅 | 1991-12-01 00:00:00 | 女 | 2 | | 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | 2 | | 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 2 | | 08 | 王菊 | 1990-01-20 00:00:00 | 女 | 0 | +-----+-------+---------------------+------+------------+
解析：
先查询总课程数，再查询所有同学的信息，筛选条件为其所学课程数小于总课程数
9、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 b.* FROM student AS b JOIN sc AS a ON b.sid = a.sid WHERE a.cid in (SELECT a.cid FROM sc AS a WHERE a.sid = '01') GROUP BY b.sid HAVING b.sid != '01';
结果：
+-----+-------+---------------------+------+ | sid | sname | sage | ssex | +-----+-------+---------------------+------+ | 02 | 钱电 | 1990-12-21 00:00:00 | 男 | | 03 | 孙风 | 1990-05-20 00:00:00 | 男 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | | 05 | 周梅 | 1991-12-01 00:00:00 | 女 | | 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | | 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | +-----+-------+---------------------+------+ 6 rows in set
解析：
#先从成绩表里查询学号为01的同学所学的课程编号，筛选条件为sc.cid in 01同学所学编号，再使用学生表和成绩表两表关联，关联字段为sid，并且把课程编号作为子查询的条件，刷选，然后再group by sid 最后通过having筛选sid 不等于01
10、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 t2.*, count(t3.cid) FROM student t2 JOIN sc t3 ON t2.sid = t3.sid WHERE t2.sid != "01" GROUP BY t2.sid HAVING count(t3.cid) = ( SELECT COUNT(*) FROM sc t1 WHERE t1.sid = "01" );
结果：
+-----+-------+---------------------+------+---------------+ | sid | sname | sage | ssex | count(t3.cid) | +-----+-------+---------------------+------+---------------+ | 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 3 | | 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 3 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | 3 | +-----+-------+---------------------+------+---------------+ 3 rows in set
解析：
先从成绩表中查询学号为01的总课程数，然后使用学生表和成绩表关联查询，关联字段为sid，消除笛卡尔积，where条件语句过滤学号01，并且用学号字段分组，并且使用having函数，统计课程总数=学号为1的课程总数
11、查询没学过"张三"老师讲授的任一门课程的学生姓名
SELECT student.sname FROM student WHERE student.sid NOT IN (SELECT sc.sid FROM sc JOIN course ON sc.cid=course.cid JOIN teacher ON course.tid=teacher.tid WHERE tname='张三' )
结果：
+-------+ | sname | +-------+ | 吴兰 | | 王菊 | +-------+ 2 rows in set
解析：
先找出所有学生选课信息及sid，再找出张三老师授课课程，将其连接，再用student里的sid not in 前面的sid
12、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 c.sname,b.* FROM student as c JOIN ( (SELECT sid ,COUNT(cid) FROM sc WHERE score < 60 GROUP BY sid HAVING COUNT(cid) >=2) as a #找出平均成绩 JOIN (SELECT sid,avg(score) FROM sc GROUP BY sid ) as b ON a.sid = b.sid) ON c.sid = b.sid
结果：
+-------+-----+------------+ | sname | sid | avg(score) | +-------+-----+------------+ | 李云 | 04 | 33.33333 | | 吴兰 | 06 | 32.50000 | +-------+-----+------------+ 2 rows in set
解析：
先查询出不及格两门或两门以上的数据，再查询出不及格的平均成绩，再三张表嵌套关联
13、查询" 01 "课程分数小于 60，按分数降序排列的学生信息
SELECT b.* ,a.score from student b JOIN (SELECT * FROM sc WHERE cid = "01" AND score < 60 ORDER BY score DESC ) as a ON a.sid = b.sid ;
结果：
+-----+-------+---------------------+------+-------+ | sid | sname | sage | ssex | score | +-----+-------+---------------------+------+-------+ | 04 | 李云 | 1990-08-06 00:00:00 | 男 | 50.0 | | 06 | 吴兰 | 1992-03-01 00:00:00 | 女 | 31.0 | +-----+-------+---------------------+------+-------+ 2 rows in set
解析：
先查询出01课程分数小于60的sid ，按照分数降序，然后和学生表关联
14、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
SELECT a.sid,a.score,a.cid,b.`平均成绩` FROM sc a JOIN (SELECT sid,avg(score) as 平均成绩 FROM sc GROUP BY sid ) as b ON a.sid = b.sid ORDER BY b.`平均成绩` DESC;
结果：
+-----+-------+-----+----------+ | sid | score | cid | 平均成绩 | +-----+-------+-----+----------+ | 07 | 89.0 | 02 | 93.50000 | | 07 | 98.0 | 03 | 93.50000 | | 01 | 80.0 | 01 | 89.66667 | | 01 | 90.0 | 02 | 89.66667 | | 01 | 99.0 | 03 | 89.66667 | | 05 | 76.0 | 01 | 81.50000 | | 05 | 87.0 | 02 | 81.50000 | | 03 | 80.0 | 01 | 80.00000 | | 03 | 80.0 | 02 | 80.00000 | | 03 | 80.0 | 03 | 80.00000 | | 02 | 70.0 | 01 | 70.00000 | | 02 | 60.0 | 02 | 70.00000 | | 02 | 80.0 | 03 | 70.00000 | | 04 | 50.0 | 01 | 33.33333 | | 04 | 30.0 | 02 | 33.33333 | | 04 | 20.0 | 03 | 33.33333 | | 06 | 31.0 | 01 | 32.50000 | | 06 | 34.0 | 03 | 32.50000 | +-----+-------+-----+----------+ 18 rows in set
解析：
先求平均成绩，注意，这里的平均成绩一定要取别名，然后取所有人的成绩，再关联，然后按照平均成绩降序排列
15、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，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
SELECT cid AS 课程id,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(sid) AS 及格率, SUM(CASE WHEN score >=70 AND score <80 THEN 1 ELSE 0 END)/count(sid) AS 中等率, SUM(CASE WHEN score >=80 AND score <90 THEN 1 ELSE 0 END)/count(sid) AS 优良率, SUM(CASE WHEN score >=90 THEN 1 ELSE 0 END)/count(sid) AS 优秀率 FROM sc GROUP BY cid ORDER BY cid ASC;
结果：
+--------+--------+--------+----------+--------+--------+--------+--------+ | 课程id | 最高分 | 最低分 | 平均分 | 及格率 | 中等率 | 优良率 | 优秀率 | +--------+--------+--------+----------+--------+--------+--------+--------+ | 01 | 80.0 | 31.0 | 64.50000 | 0.6667 | 0.3333 | 0.3333 | 0.0000 | | 02 | 90.0 | 30.0 | 72.66667 | 0.8333 | 0.0000 | 0.5000 | 0.1667 | | 03 | 99.0 | 20.0 | 68.50000 | 0.6667 | 0.0000 | 0.3333 | 0.3333 | +--------+--------+--------+----------+--------+--------+--------+--------+ 3 rows in set
解析：
重点在case when语句的用法，其实case when 就类似于 if函数 if x>某个值，then 1 else 0。就只用一个表，只是对表头需要做修改，用聚合函数+AS
16、按各科成绩进行排序，并显示排名， Score 重复时保留名次空缺
select *, rank() over(partition by cid order by score desc) AS ranked from sc;
结果：
+-----+-----+-------+--------+ | sid | cid | score | ranked | +-----+-----+-------+--------+ | 01 | 01 | 80.0 | 1 | | 03 | 01 | 80.0 | 1 | | 05 | 01 | 76.0 | 3 | | 02 | 01 | 70.0 | 4 | | 04 | 01 | 50.0 | 5 | | 06 | 01 | 31.0 | 6 | | 01 | 02 | 90.0 | 1 | | 07 | 02 | 89.0 | 2 | | 05 | 02 | 87.0 | 3 | | 03 | 02 | 80.0 | 4 | | 02 | 02 | 60.0 | 5 | | 04 | 02 | 30.0 | 6 | | 01 | 03 | 99.0 | 1 | | 07 | 03 | 98.0 | 2 | | 02 | 03 | 80.0 | 3 | | 03 | 03 | 80.0 | 3 | | 06 | 03 | 34.0 | 5 | | 04 | 03 | 20.0 | 6 | +-----+-----+-------+--------+ 18 rows in set
解析：
MySQL可以实现Oracle中的排名公式，一共有三种
1. rank() over(order by col_name desc）2.dense_rank() over() 3.row_number() over()
第一个是如果出现了相同排名都为同一排名，下个排名跳过，例如1,1,3,4
第二个是如果出现了相同排名都为同一排名，下个排名不跳过，例如1,1,2,3
第三个是直接对行进行排名不分是否有相同值
此题目要按照各科成绩进行排序 over()中要填partition by col_name order by col_name
第一个colname 为分组的内容，第二个是按什么值排的内容
17、查询学生的总成绩，并进行排名，总分重复时保留名次空缺
SELECT a.*,rank() over(ORDER BY a.总成绩 DESC) AS Ranked FROM (SELECT *, SUM(score) AS 总成绩 FROM sc GROUP BY sid) AS a ;
结果：
+-----+-----+-------+--------+--------+ | sid | cid | score | 总成绩 | Ranked | +-----+-----+-------+--------+--------+ | 01 | 01 | 80.0 | 269.0 | 1 | | 03 | 01 | 80.0 | 240.0 | 2 | | 02 | 01 | 70.0 | 210.0 | 3 | | 07 | 02 | 89.0 | 187.0 | 4 | | 05 | 01 | 76.0 | 163.0 | 5 | | 04 | 01 | 50.0 | 100.0 | 6 | | 06 | 01 | 31.0 | 65.0 | 7 | +-----+-----+-------+--------+--------+ 7 rows in set
解析：
跟上题一样用rank（）over（），只是多了层嵌套
18、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.*,dense_rank() over(ORDER BY a.total_socre DESC) AS Ranked FROM (SELECT *,SUM(score) AS total_socre FROM sc GROUP BY sid) AS a;
结果：
+-----+-----+-------+-------------+--------+ | sid | cid | score | total_socre | Ranked | +-----+-----+-------+-------------+--------+ | 01 | 01 | 80.0 | 269.0 | 1 | | 03 | 01 | 80.0 | 240.0 | 2 | | 02 | 01 | 70.0 | 210.0 | 3 | | 07 | 02 | 89.0 | 187.0 | 4 | | 05 | 01 | 76.0 | 163.0 | 5 | | 04 | 01 | 50.0 | 100.0 | 6 | | 06 | 01 | 31.0 | 65.0 | 7 | +-----+-----+-------+-------------+--------+ 7 rows in set
解析：
和上面一样，只是换成dense_rank () over()，只是总分没有重复无法看出区别
19、统计各科成绩各分数段人数：课程编号，[100-85)，[85-70)，[70-60)，[60-0] 及所占百分比
SELECT cid AS 课程ID, SUM(CASE WHEN score <= 60 THEN 1 ELSE 0 END)/count(sid) AS 百分比1, SUM(CASE WHEN score >60 AND score <=70 THEN 1 ELSE 0 END)/count(sid) AS 百分比2, SUM(CASE WHEN score >70 AND score <=85 THEN 1 ELSE 0 END)/count(sid) AS 百分比3, SUM(CASE WHEN score >85 THEN 1 ELSE 0 END)/count(sid) AS 百分比4 FROM sc GROUP BY cid ORDER BY cid
结果：
+--------+---------+---------+---------+---------+ | 课程ID | 百分比1 | 百分比2 | 百分比3 | 百分比4 | +--------+---------+---------+---------+---------+ | 01 | 0.3333 | 0.1667 | 0.5000 | 0.0000 | | 02 | 0.3333 | 0.0000 | 0.1667 | 0.5000 | | 03 | 0.3333 | 0.0000 | 0.3333 | 0.3333 | +--------+---------+---------+---------+---------+ 3 rows in set
解析：
使用case when
20、查询各科成绩前三名的记录
SELECT * FROM (SELECT *,rank() over(PARTITION by cid ORDER BY score desc) as ranked FROM sc) as a WHERE a.ranked <=3;
结果：
+-----+-----+-------+--------+ | sid | cid | score | ranked | +-----+-----+-------+--------+ | 01 | 01 | 80.0 | 1 | | 03 | 01 | 80.0 | 1 | | 05 | 01 | 76.0 | 3 | | 01 | 02 | 90.0 | 1 | | 07 | 02 | 89.0 | 2 | | 05 | 02 | 87.0 | 3 | | 01 | 03 | 99.0 | 1 | | 07 | 03 | 98.0 | 2 | | 02 | 03 | 80.0 | 3 | | 03 | 03 | 80.0 | 3 | +-----+-----+-------+--------+ 10 rows in set
解析：
与上面rank一样，用rank（）over（）where ranked <=3
注意！where 的执行顺序在select前，嵌套一个select 语句就好
21、查询每门课程被选修的学生数
SELECT cid AS 课程id,COUNT(sid) AS 选修的学生数 FROM sc GROUP BY cid ORDER BY 课程id;
结果：
+--------+--------------+ | 课程id | 选修的学生数 | +--------+--------------+ | 01 | 6 | | 02 | 6 | | 03 | 6 | +--------+--------------+ 3 rows in set
解析：
单表 查询，使用group by ，order by
22、查询出只选修两门课程的学生学号和姓名
SELECT student.sname,a.* FROM student JOIN (SELECT sid,count(cid) as 选修课程数 FROM sc GROUP BY sid HAVING 选修课程数 = 2) as a ON student.sid = a.sid;
结果：
+-------+-----+------------+ | sname | sid | 选修课程数 | +-------+-----+------------+ | 周梅 | 05 | 2 | | 吴兰 | 06 | 2 | | 郑竹 | 07 | 2 | +-------+-----+------------+ 3 rows in set
解析：
先从成绩表中查询出只选修两门课程的学生id和课程数，再和学生表进行关联查询
备注：
众所周知SQL的执行顺序应该为
FORM-JOIN ON-WHERE-GROUP BY-HAVING-SELECT-DISTINCT-UNION-ORDER
但是为什么上述语句中的能够在HAVING后面直接更SELECT语句里面的别名
因为mysql在版本迭代中，mysql对having做出了扩展，即having能够'解析'select中的别名，但是执行顺序还是having在select前。但是大多数标准SQL中，having是无法引用别名的。
还有where 后不能用聚合函数，having后可以用聚合函数
23、查询男生、女生人数
SELECT ssex,COUNT(sid) FROM student GROUP BY ssex;
结果：
+------+------------+ | ssex | COUNT(sid) | +------+------------+ | 男 | 4 | | 女 | 4 | +------+------------+ 2 rows in set
解析：
根据ssex group by后再count()
24、查询名字中含有「风」字的学生信息
SELECT * FROM student WHERE sname like "%风%";
结果：
+-----+-------+---------------------+------+ | sid | sname | sage | ssex | +-----+-------+---------------------+------+ | 03 | 孙风 | 1990-05-20 00:00:00 | 男 | +-----+-------+---------------------+------+ 1 row in set
解析：
通配符，%，‘%a’a结尾，‘a%’a开头，‘%a%’含有a
25、查询同名同性学生名单，并统计同名人数
SELECT *,COUNT(sid) as 同名人数 FROM (SELECT a.* FROM student AS a JOIN student as b WHERE a.sname = b.sname AND a.ssex = b.ssex) as c GROUP BY sid HAVING 同名人数 >=2;
结果：
解析：
连接表student和student on ssname and ssex 在group by sid（因为id唯一，name可能重名），count sid
26、查询 1990 年出生的学生名单
SELECT * FROM student WHERE YEAR(sage) = 1990;
结果：
+-----+-------+---------------------+------+ | 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 | 1990-05-20 00:00:00 | 男 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | | 08 | 王菊 | 1990-01-20 00:00:00 | 女 | +-----+-------+---------------------+------+ 5 rows in set
解析：
sage一列为datetime类型，用时间函数。MySQL里面能够对datetime类型函数截取年、月、周、日等等 ，用YEAR()来表示年，以此类推
27、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) AS 平均成绩 FROM sc GROUP BY cid ORDER BY 平均成绩 DESC,cid ASC;
结果：
+-----+----------+ | cid | 平均成绩 | +-----+----------+ | 02 | 72.66667 | | 03 | 68.50000 | | 01 | 64.50000 | +-----+----------+ 3 rows in set
解析：
order by x desc,y,z,... 先根据x排序，再根据y，然后z....
28、 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 student.sname , a.* FROM student JOIN (SELECT sid as 学号, avg(score) as 平均成绩 FROM sc GROUP BY sid HAVING 平均成绩 > 85) as a ON student.sid = a.学号;
结果：
+-------+------+----------+ | sname | 学号 | 平均成绩 | +-------+------+----------+ | 赵雷 | 01 | 89.66667 | | 郑竹 | 07 | 93.50000 | +-------+------+----------+ 2 rows in set
解析：
先从成绩表中查询出平均成绩大于85的学生好和平均成绩（记住，这里需要取别名），然后再和学生表关联，关联字段为sid，获取到学生名字
29、查询课程名称为「数学」，且分数低于 60 的学生姓名和分数
SELECT student.sname,c.* FROM student JOIN (SELECT t1.cname,t2.score,t2.sid FROM course as t1 JOIN sc as t2 ON t1.cid = t2.cid WHERE t2.score < 60 AND t1.cname = "数学") as c ON student.sid = c.sid;
结果：
+-------+-------+-------+-----+ | sname | cname | score | sid | +-------+-------+-------+-----+ | 李云 | 数学 | 30.0 | 04 | +-------+-------+-------+-----+ 1 row in set
解析：
先把课程表和成绩表关联，获取到低于60分的学生号、分数和课程名称，作为临时表，然后再和学生表关联，获取到最后一个字段，学生姓名
30、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 student.sname,c.* FROM student JOIN (SELECT a.cname,b.sid,b.score FROM course as a LEFT JOIN sc AS b on a.cid = b.cid) as c on student.sid = c.sid;
结果：
+-------+-------+-----+-------+ | sname | cname | sid | score | +-------+-------+-----+-------+ | 赵雷 | 语文 | 01 | 80.0 | | 赵雷 | 数学 | 01 | 90.0 | | 赵雷 | 英语 | 01 | 99.0 | | 钱电 | 语文 | 02 | 70.0 | | 钱电 | 数学 | 02 | 60.0 | | 钱电 | 英语 | 02 | 80.0 | | 孙风 | 语文 | 03 | 80.0 | | 孙风 | 数学 | 03 | 80.0 | | 孙风 | 英语 | 03 | 80.0 | | 李云 | 语文 | 04 | 50.0 | | 李云 | 数学 | 04 | 30.0 | | 李云 | 英语 | 04 | 20.0 | | 周梅 | 语文 | 05 | 76.0 | | 周梅 | 数学 | 05 | 87.0 | | 吴兰 | 语文 | 06 | 31.0 | | 吴兰 | 英语 | 06 | 34.0 | | 郑竹 | 数学 | 07 | 89.0 | | 郑竹 | 英语 | 07 | 98.0 | +-------+-------+-----+-------+ 18 rows in set
解析：
先把课程表和成绩表关联，关联字段为cid，获取到课程名称，学生号和学科成绩，作为临时表，然后再和学生表关联，关联字段为sid，获取到学生名字
31、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 student.sname,c.* FROM student JOIN (SELECT a.cname,b.sid,b.score FROM course as a LEFT JOIN sc AS b on a.cid = b.cid) as c on student.sid = c.sid WHERE c.score > 70;
结果：
+-------+-------+-----+-------+ | sname | cname | sid | score | +-------+-------+-----+-------+ | 赵雷 | 语文 | 01 | 80.0 | | 赵雷 | 数学 | 01 | 90.0 | | 赵雷 | 英语 | 01 | 99.0 | | 钱电 | 英语 | 02 | 80.0 | | 孙风 | 语文 | 03 | 80.0 | | 孙风 | 数学 | 03 | 80.0 | | 孙风 | 英语 | 03 | 80.0 | | 周梅 | 语文 | 05 | 76.0 | | 周梅 | 数学 | 05 | 87.0 | | 郑竹 | 数学 | 07 | 89.0 | | 郑竹 | 英语 | 07 | 98.0 | +-------+-------+-----+-------+ 11 rows in set
解析：
在上一题的基础上增加score > 70,使用where 或and都可以
32、查询不及格的课程
SELECT cname,a.* FROM course JOIN (SELECT score,cid FROM sc WHERE score < 60) as a ON course.cid = a.cid;
结果：
+-------+-------+-----+ | cname | score | cid | +-------+-------+-----+ | 语文 | 50.0 | 01 | | 数学 | 30.0 | 02 | | 英语 | 20.0 | 03 | | 语文 | 31.0 | 01 | | 英语 | 34.0 | 03 | +-------+-------+-----+ 5 rows in set
解析：
先从成绩表中获取到不及格的课程id和成绩，然后再和课程表关联，关联字典为课程id，获取到课程名称
33、查询课程编号为 01 且课程成绩在 60 分以上的学生的学号和姓名
SELECT student.sname,c.* FROM student JOIN (SELECT b.sid ,b.score,a.cid ,a.cname FROM course as a JOIN sc as b ON a.cid = b.cid WHERE a.cid = "01" AND b.score > 60) as c ON student.sid = c.sid;
结果：
+-------+-----+-------+-----+-------+ | sname | sid | score | cid | cname | +-------+-----+-------+-----+-------+ | 赵雷 | 01 | 80.0 | 01 | 语文 | | 钱电 | 02 | 70.0 | 01 | 语文 | | 孙风 | 03 | 80.0 | 01 | 语文 | | 周梅 | 05 | 76.0 | 01 | 语文 | +-------+-----+-------+-----+-------+ 4 rows in set
解析：
先从课程表和成绩表中获取到学生号、成绩、课程号和课程名称，关联字段为课程号，作为临时表，然后再和学生表关联，关联字段为学生号，获取到学生名字
34、求每门课程的学生人数
SELECT course.cname,a.* FROM course JOIN (SELECT count(sid),cid FROM sc GROUP BY cid) as a ON course.cid = a.cid;
结果：
+-------+------------+-----+ | cname | count(sid) | cid | +-------+------------+-----+ | 语文 | 6 | 01 | | 数学 | 6 | 02 | | 英语 | 6 | 03 | +-------+------------+-----+ 3 rows in set
解析：
先从成绩表中统计出每门课程的人数，再和课程表关联，关联字段为课程号，获取到课程名称
35、成绩没有重复的情况下，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
SELECT student.sname ,e.* FROM student JOIN (SELECT MAX(d.score),c.* ,d.sid FROM sc AS d JOIN (SELECT a.tid,a.tname,b.cid,b.cname FROM teacher AS a JOIN course AS b ON a.tid = b.tid WHERE a.tname = "张三") as c ON d.cid = c.cid) as e ON student.sid = e.sid;
结果：
+-------+--------------+-----+-------+-----+-------+-----+ | sname | MAX(d.score) | tid | tname | cid | cname | sid | +-------+--------------+-----+-------+-----+-------+-----+ | 赵雷 | 90.0 | 01 | 张三 | 02 | 数学 | 01 | +-------+--------------+-----+-------+-----+-------+-----+ 1 row in set
解析：
教师表和课程表关联，获取到教师编号、教师名称和课程编号和课程名称，关联字段为教师编号
作为临时表再和成绩表关联，关联字段为课程编号
作为临时表再和学生表关联，关联字段为学生号
36、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
SELECT student.sname ,e.* FROM student JOIN (SELECT MAX(d.score),c.* ,d.sid,rank() over(ORDER BY MAX(d.score))as Ranked FROM sc AS d JOIN (SELECT a.tid,a.tname,b.cid,b.cname FROM teacher AS a JOIN course AS b ON a.tid = b.tid WHERE a.tname = "张三") as c ON d.cid = c.cid) as e ON student.sid = e.sid WHERE e.Ranked ;
结果：
+-------+--------------+-----+-------+-----+-------+-----+--------+ | sname | MAX(d.score) | tid | tname | cid | cname | sid | Ranked | +-------+--------------+-----+-------+-----+-------+-----+--------+ | 赵雷 | 90.0 | 01 | 张三 | 02 | 数学 | 01 | 1 | +-------+--------------+-----+-------+-----+-------+-----+--------+ 1 row in set
解析：
用rank函数，然后再嵌套一个select，where rank = 1
37、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 DISTINCT a.* FROM sc AS a JOIN sc AS b ON a.score =b.score AND a.cid != b.cid
结果：
+-----+-----+-------+ | sid | cid | score | +-----+-----+-------+ | 02 | 03 | 80.0 | | 03 | 02 | 80.0 | | 03 | 03 | 80.0 | | 01 | 01 | 80.0 | | 03 | 01 | 80.0 | +-----+-----+-------+ 5 rows in set
解析：
sc表自连，distinct去重，cid 不同，score相同
38、查询每门功成绩最好的前两名
SELECT * FROM (SELECT *,dense_rank()over(PARTITION BY cid ORDER BY score DESC) AS ranked FROM sc ) a WHERE a.ranked <=2
结果：
+-----+-----+-------+--------+ | sid | cid | score | ranked | +-----+-----+-------+--------+ | 01 | 01 | 80.0 | 1 | | 03 | 01 | 80.0 | 1 | | 05 | 01 | 76.0 | 2 | | 01 | 02 | 90.0 | 1 | | 07 | 02 | 89.0 | 2 | | 01 | 03 | 99.0 | 1 | | 07 | 03 | 98.0 | 2 | +-----+-----+-------+--------+ 7 rows in set
解析：
我认为最好的前两名是排名的前2个，即第一个排名1 和第二个排名2，如果有两个并列第一，一个第二，那么前两名应该是3个人，用dense_rank，排名不跳过；如果说是最好的前两个人，就用rank，排名跳过
39、统计每门课程的学生选修人数（超过 5 人的课程才统计）
SELECT course.cname,a.* FROM course JOIN (SELECT cid,COUNT(sid) as 选修人数 FROM sc GROUP BY cid HAVING COUNT(sid) >5) as a ON course.cid = a.cid;
结果：
+-------+-----+----------+ | cname | cid | 选修人数 | +-------+-----+----------+ | 语文 | 01 | 6 | | 数学 | 02 | 6 | | 英语 | 03 | 6 | +-------+-----+----------+ 3 rows in set
解析：
group by，having聚合
40、检索至少选修两门课程的学生学号
SELECT student.sname,a.* FROM student JOIN (SELECT sid,COUNT(cid) as 选修课程总数 FROM sc GROUP BY sid HAVING 选修课程总数 >=2) as a on student.sid = a.sid;
结果：
+-------+-----+--------------+ | sname | sid | 选修课程总数 | +-------+-----+--------------+ | 赵雷 | 01 | 3 | | 钱电 | 02 | 3 | | 孙风 | 03 | 3 | | 李云 | 04 | 3 | | 周梅 | 05 | 2 | | 吴兰 | 06 | 2 | | 郑竹 | 07 | 2 | +-------+-----+--------------+ 7 rows in set
解析：
41、查询选修了全部课程的学生信息
SELECT student.*,c.`选修课程总数` from student JOIN (SELECT b.sid,COUNT(a.cid) as 选修课程总数 FROM course a JOIN sc b ON a.cid = b.cid GROUP BY b.sid HAVING COUNT(a.cid) = (SELECT COUNT(cid) FROM course)) as c ON student.sid = c.sid;
结果：
+-----+-------+---------------------+------+--------------+ | sid | sname | sage | ssex | 选修课程总数 | +-----+-------+---------------------+------+--------------+ | 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 3 | | 02 | 钱电 | 1990-12-21 00:00:00 | 男 | 3 | | 03 | 孙风 | 1990-05-20 00:00:00 | 男 | 3 | | 04 | 李云 | 1990-08-06 00:00:00 | 男 | 3 | +-----+-------+---------------------+------+--------------+ 4 rows in set
解析：
从课程表中查询出总的课程数，作为后面子查询的条件
从成绩表中查询出选修了全部课程数的的学生号和选修的课程总数
作为临时表和学生表关联，关联字段为学生号，获取到全部的学生信息
42、查询各学生的年龄，只按年份来算
SELECT sname,YEAR(NOW()) - YEAR(sage) as 年龄 FROM student;
结果：
+-------+------+ | sname | 年龄 | +-------+------+ | 赵雷 | 31 | | 钱电 | 31 | | 孙风 | 31 | | 李云 | 31 | | 周梅 | 30 | | 吴兰 | 29 | | 郑竹 | 32 | | 王菊 | 31 | +-------+------+ 8 rows in set
解析：
使用year函数
43、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 sname, CASE WHEN (DATE_FORMAT(NOW(),'%m-%d') - DATE_FORMAT(sage,'%m-%d')) < 0 THEN YEAR(NOW()) - YEAR(sage) - 1 ELSE YEAR(NOW()) - YEAR(sage) END AS age FROM student;
结果：
+-------+-----+ | sname | age | +-------+-----+ | 赵雷 | 31 | | 钱电 | 30 | | 孙风 | 31 | | 李云 | 31 | | 周梅 | 29 | | 吴兰 | 29 | | 郑竹 | 32 | | 王菊 | 31 | +-------+-----+ 8 rows in set
解析：
有两种方法，一种是利用date_format直接截取时间类型中的月日，直接比大小
另外一种是用month()先比大小，相等再用day()比大小
44、查询本周过生日的学生
SELECT sname FROM student WHERE week(NOW()) = WEEK(sage);
结果：
Empty set
解析：
week() 返回的是今年的第几周，即如果本周过生，返回数字相等
45、查询下周过生日的学生
SELECT sname FROM student WHERE week(NOW()) + 1 = WEEK(sage);
结果：
Empty set
解析：
加一就行
46、查询本月过生日的学生
SELECT sname FROM student WHERE month(NOW()) = month(sage);
结果：
Empty set
解析：
使用month函数
47、查询下月过生日的学生
SELECT sname FROM student WHERE month(NOW()) + 1 = month(sage);
结果：
Empty set
解析：
加一即可
48、49、50是删除的，这里就不罗列