二、SQL系列之~常见51道SQL查询语句

【写在前面~~】

【PS1：建议SQL初学者一定要自己先做一遍题目，这样才有效果~~（做题时为验证查询结果是否正确，可更改表中数据）】

【PS2：文末最后一条代码整合了全部51道题目及答案~~】

【PS3：题目是网上所找，文中代码为自己练习编写，若有错误，请尽情指出~~】

一、创建数据库

 create database 测试1 on primary
 (name='测试1.mdf',
 filename='E:\学习\SQL\实例目录\DATA\测试1.mdf',
 SIZE=5MB,
 MAXSIZE=UNLIMITED,
 FILEGROWTH=1KB)
  LOG ON
  (NAME='测试1.ldf',
  filename='E:\学习\SQL\实例目录\DATA\测试1.ldf',
  size=520kb,
  maxsize=unlimited,
  filegrowth=20kb)

二、创建表

student （学生表）

列名	类型	是否为空值	键约束
sid (学生编号)	int	否	主键
sname (学生姓名)	varchar(10)
sage( 出生年月)	datetime
ssex (性别)	varchar(10)

teacher （教师表）

列名	类型	是否为空值	键约束
tid (教师编号)	int	否	主键
tname (教师姓名)	varchar(10)

course （课程表）

列名	类型	是否为空值	键约束
cid (课程编号)	int	否	主键
cname (课程名)	varchar(10)
tid (教师编号)	int		外键

sc(成绩表)

列名	类型	是否为空值	键约束
sid (学生编号)	int		外键
cid (课程编号)	int		外键
score (分数)	decimal(5,2)

 use 测试1
 go
 create table student
 (sid varchar(10) not null constraint pk_sid  primary key,
 sname varchar(10),
 sage datetime,
 ssex  varchar(10) constraint ck_ssex check  (ssex in ('男','女')))--设置检查约束，性别一列只能输入男或女

 create table teacher
 (tid varchar(10) not null constraint pk_tid primary key ,
 tname varchar(10))

 create table course
 (cid varchar(10) not null constraint pk_cid primary key,
 cname varchar(10),
 tid  varchar(10) constraint fk_tid foreign key (tid) references teacher (tid))--tid 作为外键，引用teacher表中的主键tid

 create table sc
 (sid varchar(10) constraint fk_sid foreign key (sid) references student(sid),
 cid varchar(10) constraint fk_cid foreign key(cid) references course(cid),
 score decimal(5,2))

三、向表中插入数据

 insert into student values('' , '赵雷' , '1990-01-01' , '男')
 insert into student values('' , '钱电' , '1990-12-21' , '男')
 insert into student values('' , '孙风' , '1990-06-20' , '男')
 insert into student values('' , '李云' , '1990-08-06' , '男')
 insert into student values('' , '周梅' , '1991-12-01' , '女')
 insert into student values('' , '吴兰' , '1992-03-01' , '女')
 insert into student values('' , '郑竹' , '1989-06-04' , '女')
 insert into student values('' , '王菊' , '1990-05-28' , '女')
 insert into student values('' , '风王' , '1989-05-29' , '男')
 insert into student values('' , '司徒风风' , '1991-06-05' , '女')
 insert into student values('' , '桂易' , '1999-06-21' , '男')
 insert into student values('' , '司徒末' , '1991-07-30' , '女')
 insert into student values('' , '风王' , '1995-05-30' , '男')
 insert into student values('' , '顾唯一' , '1998-06-02' , '男')
 insert into student values('' , '圆圆' , '2000-06-06' , '女')
 insert into student values('' , '司徒叮当' , '1997-06-03' , '女')
 insert into student values('' , '顾未易' , '1999-06-12' , '男')
 insert into student values('' , '顾未易' , '1999-06-08' , '男')

 insert into teacher values('' , '张三')
 insert into teacher values('' , '李四')
 insert into teacher values('' , '王五')

 insert into course values('' , '语文' , '')
 insert into course values('' , '数学' , '')
 insert into course values('' , '英语' , '')
 insert into course values('' , '政治' , '')
 insert into course values('' , '物理' , '')

 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 90)
 insert into SC values('' , '' , 99)
 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 60)
 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 80)
 insert into SC values('' , '' , 30)
 insert into SC values('' , '' , 90)
 insert into SC values('' , '' , 76)
 insert into SC values('' , '' , 87)
 insert into SC values('' , '' , 31)
 insert into SC values('' , '' , 34)
 insert into SC values('' , '' , 90)
 insert into SC values('' , '' , 98)
 insert into SC values('' , '' , 90)
 insert into SC values('' , '' , 50)
 insert into SC values('' , '' , 60)

   select student.*,course.*,teacher.tname,sc.score  into newtable
    from student left join sc  on student.sid=sc.sid left join course on course.cid= sc.cid left join teacher on teacher.tid=course.tid --将所有信息合在一起，为了后面查看校验查询信息是否正确
 --若插入数据时重复执行了（若没有设置主键约束情况下）,导致在删除数据时无法删除,提示已更新或删除行值要么不能使该行成为唯一行,要么...,则可用临时表来实现删除重复数据
 select 各列字段名或全部列用*表示 into #临时表名 from 原表名 group by  字段列表
 truncate table 原表名
 insert into 原表名  select*from #临时表名
 drop table #临时表名

四、51道题目及答案

--1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数

 --第一种方法
 select student.*,m.score [01课程成绩], n.score [02课程成绩]from student
   join (select sid,score from sc where cid='') m on student.sid=m.sid
   join (select sid,score from sc where cid='')n  on student.sid=n.sid
 where m.score>n.score
 --第二种方法
 select student.*,m.[01课程成绩],m.[02课程成绩] from student,
 (select sid,sum(case when cid='' then score else null end) [01课程成绩] ,--若else后面是0,则结果会输出有01课程无02课程的学生,若改为null则不会输出只有一门课程的学生成绩
 sum(case when cid='' then score else null end)  [02课程成绩]from sc group by sid) m
 where student.sid=m.sid and m.[01课程成绩]>m.[02课程成绩]

--2、查询同时存在"01"课程和"02"课程的情况

   select student.*,m.score [01课程成绩],n.score [02课程成绩] from student
   join (select sid ,score from sc where cid='') m on  student.sid=m.sid
 join  (select sid,score from sc where cid='') n on student.sid=n.sid

--3、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 student.*,m.score,n.score  from (select sid,score from sc where cid='') m left join student on student.sid=m.sid
   left join (select sid,score from sc where cid='')n on student.sid=n.sid

--4、 查询不存在" 01 "课程但存在" 02 "课程的情况

 select student.*,score from student join sc on student.sid=sc.sid where  cid='' and student.sid not in (select sid from sc where cid='')

--5、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 student.sid,sname,cast (avg(score)  as decimal(5,2))平均成绩 from student join sc on student.sid=sc.sid
 group by student.sid,sname having avg(score)>=60

--6、查询在sc表存在成绩的学生信息的SQL语句

 --第一种
 select distinct student.* from student join sc on student.sid=sc.sid
 --第二种
 select * from student where sid in (select sc.sid from sc )--IN()适合B表(括号内的表是B)比A表数据小的情况
 --第三种
 select * from student where exists (select sc.sid from sc where student.sid = sc.sid)--EXISTS()适合B表比A表数据大的情况

--7、查询在sc表中不存在成绩的学生信息的SQL语句

 select * from student where sid  not in (select sc.sid from sc )

--8、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(没成绩的显示为 null )

  select student.sid,sname,count(cid) 选课总数 ,sum(score ) 所有课程成绩的总和 from student
  left join sc on student.sid=sc.sid group by student.sid,sname

--9、查询「李」姓老师的数量

  select count (tid) 李姓老师的数量  from teacher where tname like '李%'

--10、查询学过「张三」老师授课的同学的信息

 select student.* from student join sc on student.sid=sc.sid
 join course on course.cid=sc.cid
 join teacher on teacher.tid=course.tid  where tname='张三'

--11、查询没学过「张三」老师授课的同学的信息

 select  *from student where sid not in (select sc.sid from sc join course on course.cid=sc.cid
 join teacher on teacher.tid=course.tid  where tname='张三')

--12、查询没有学全所有课程的同学的信息

 --第一种直接
 select student.*  from student left  join sc on student.sid=sc.sid group by student.sid,sname,sage,ssex
 having count(sc.cid)<(select count(course.cid) from course)
 ----第二种反向
 select student.* from student where student.sid not in
 (select sc.sid from sc group by sid having count(sc.cid)= (select count(cid) from course)) 

--13、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 student.* from student  join sc on student.sid=sc.sid
  where cid in (select cid from sc where sid='') and student.sid!='' --in的意思是:等于其中的一个则输出该条相等数据，直至全部输出为止

--14、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
--(即所学课程数要一致,学的课程目录也要一致)

 select student.* from student join
 (select sid from sc  group  by sid having  count(cid)=(select count(cid) from sc where sid='' )) m--控制大范围下学生所学课程数=01同学的课程数3个,例如:学4门课程的同学,其中3门与01同学相同,也满足后续程序,但是与01同学就不是完全相同的.
  on student.sid=m.sid  join sc on student.sid=sc.sid
 where sc.cid in (select cid from sc where sid='') and student.sid!=''group by student.sid,sname,sage,ssex
 having count(sc.cid)=(select count(sc.cid) from sc where sid='')--控制与01同学所学课程目录相同的数也是3个

--15、查询没学过"张三"老师讲授的任一门课程的学生姓名

 select * from student where student.sid not in
 (select sc.sid from sc join course on sc.cid=course.cid
 join teacher on teacher.tid=course.tid where  tname='张三' )

--16、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

 --第一种
 select student.sid,sname,cast(avg(score) as decimal(5,2)) avgscore from student  join sc
 on student.sid=sc.sid where sc.score<60 group by student.sid,sname having count(sc.cid)>=2
 --第二种
 select student.sid,sname,cast(avg(score) as decimal(5,2)) avgscore from student join sc
 on student.sid=sc.sid group by student.sid,sname having sum(case when score<60 then 1 else 0 end)>=2

--17、检索"01"课程分数小于60，按分数降序排列的学生信息

 select student.* from student join sc on student.sid=sc.sid where cid='' and score<60 order by score desc

--18、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 student.sid,sname, sc.cid,sc.score,m.avgscore from student left join sc on student.sid=sc.sid
  left join (select sid,cast(avg(score) as decimal(5,2)) avgscore from sc group by sid ) m on sc.sid=m.sid
  order by m.avgscore desc

--19、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 sc.cid,course.cname,max(sc.score) 最高分,min(sc.score) 最低分,cast(avg(sc.score) as decimal(5,2)) 平均分 ,
  count(sc.cid) 计数,
  sum(case when sc.score>=60 then 1 else 0 end )及格数, sum(case when sc.score>=70 and sc.score<80 then 1 else 0 end ) 中等数,
   sum(case when sc.score>=80 and sc.score<90 then 1 else 0 end ) 优良数,sum(case when sc.score>=90 then 1 else 0 end ) 优秀数,--这里的计数,及格数...优秀数是为了验证后续的及格率...优秀率是否正确的,完全按照题意需省去
  convert(varchar(10),cast((100*sum(case when score>=60 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 及格率,--分母count(sc.sid)要转化为浮点型数据，否则分子是整数，结果也为整数，舍小数位或者把分子转化为浮点型数据，结果也会为浮点型
  convert(varchar(10),cast((100*sum(case when sc.score>=70 and sc.score<80 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 中等率,
  convert(varchar(10),cast((100*sum(case when sc.score>=80 and sc.score<90 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 优良率,
  convert(varchar(10),cast((100*sum(case when sc.score>=90 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 优秀率
  from course join sc on course.cid=sc.cid group by sc.cid,course.cname

--20、按各科成绩进行排序，并显示排名,score 重复时保留名次空缺

 --第一种
  select a.cid,a.sid,a.score,count(b.score) +1 排名 from sc a
  left join sc b on a.cid=b.cid and a.score<b.score  group by a.cid,a.sid,a.score order by a.cid asc,排名 asc
  ---select a.cid,a.sid,a.score,b.score from sc a left join sc b on a.cid=b.cid and a.score<b.score group by a.cid,a.sid,a.score,b.score 在课程相同的情况下，计算比自己分数大的有几个再加上1即为排名
  --例如课程3,学号1,99；2,80；3,80；4,90；6,34;7,98
  --则对应的比1,99大的为null排名1;比2,80大的为99,90,98排名4;比3,80大的99,90,98排名3;比4,90大的为99,98排名3；比6,34大的为99,90,98,80,80排名6；比7,98大的为99排名2

  --第二种(使用内置函数rank)
  select cid,sid,score,rank() over (partition  by cid order by score desc )  排名 from sc --partition by 分组 order by 排序

--21、按各科成绩进行排序，并显示排名,score 重复时合并名次空缺

    --第一种
  select a.cid,a.sid,a.score,count(distinct b.score) +1 排名 from sc a
  left join sc b on a.cid=b.cid and a.score<b.score  group by a.cid,a.sid,a.score order by a.cid asc,排名 asc
   --第二种(使用内置函数dense_rank)
  select cid,sid,score,dense_rank() over (partition  by cid order by score desc )  排名 from sc

--22、查询学生的总成绩，并进行排名，总分重复时合并名次空缺

 --第一种
 select a.sid,a.allscore 总成绩,count(distinct b.allscore)+1 排名 from ( select sid,sum(score) allscore from sc group by sid) a
 left join ( select sid,sum(score) allscore from sc group by sid) b on a.allscore<b.allscore group by a.sid,a.allscore order by 排名 asc
 --a表与b表中已经进行完全相同的学生sid分组，在左连接时不应再让sid相等连接，否则左连接时b.allscore >a.allscore匹配为空值，每个学生的排名都为1，即在学号sid相等情况，不能再比较不同学生间的成绩排名,只能自我比较
 --而20/21题的第一种方法有cid左连接相等即在同一课程下，还可以比较不同学生sid间的成绩排名
 --第二种
 select sid,sum(score) 总成绩 ,dense_rank()over(order by sum(score) desc ) 排名 from sc group by sid
 --这里的成绩不是所有同学sid的成绩，有的同学没有成绩，因此我们可以将student与上述两种方法进行左连接--例如：
 select a.sid ,a.allscore 总成绩,count(distinct b.allscore)+1 排名 from
 (select student.sid ,isnull(sum(score),0) allscore from student left join sc on student.sid=sc.sid group by student.sid ) a left join
 (select student.sid ,isnull(sum(score),0) allscore from student left join sc on student.sid=sc.sid group by student.sid ) b on a.allscore<b.allscore
 group by a.sid,a.allscore order by 排名 asc

 select student.sid,isnull(sum(score),0) 总成绩,dense_rank()over(order by isnull(sum(score),0) desc )排名
 from student left join sc on student.sid=sc.sid group by student.sid

--23、查询不同老师所教不同课程平均分从高到低显示

  select course.tid,tname,course.cid,cast(avg(score) as decimal(5,2)) 课程平均分 from course left join sc   on  sc.cid=course.cid
 left join teacher on teacher.tid=course.tid  group by course.tid ,tname,course.cid order by avg(score) desc

--24、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

 --第一种
 select student.*,m.cid,m.成绩,m.排名 from student left join
 ( select  a.cid,a.sid,a.score 成绩,count(b.score)+1 排名 from sc a left join sc b on a.cid=b.cid and a.score<b.score
 group by a.cid,a.sid,a.score ) m on m.sid=student.sid where m.排名 between 2 and 3 order by m.cid asc,排名 asc--重复排名时保留名次空缺,把count 内改成distinct b.score 即为重复时合并空缺
 --第二种
 select student.*,m.cid,m.成绩,m.排名 from student left join
 (select cid,sid,score 成绩,rank()over(partition by cid order by score desc) 排名 from sc group by cid,sid,score) m
 on student.sid=m.sid  where m.排名 between 2 and 3 order by cid asc--rank 改为dense_rank 即为合并名次空缺

--25、查询各科成绩前三名的记录

 --第一种
 select a.cid,a.sid,a.score from sc a left join sc b on a.cid=b.cid and a.score<b.score
 group by a.cid,a.sid,a.score having  count(b.score)<3 order  by a.cid asc ,a.score desc --成绩重复时保留名次空缺，输出排名为1-3的学生;也可以按照24题来做,排名改成between 1 and 3即可
 --第二种
 select cid,sid,score from sc a where  sid in  (
 select  top 3 b.sid  from sc b where a.cid=b.cid order by b.cid asc ,b.score desc )
 order by  cid asc,score  desc --这种top 型的只输出前3个学生，对于成绩重复的同一名次会缺失
 --第三种
 select * from
 (select *,row_number()over(partition by cid order by score desc) 排名 from sc) B
 where B.排名<4--同样只输出前3个学生，对于成绩重复的同一名次会缺失

--26、查询各科成绩最高的记录

 select sc.cid, sc.sid,m.maxscore from sc  join (select cid,max(score) maxscore from sc group by cid ) m
 on m.maxscore=sc.score and   m.cid=sc.cid
 --其余方法可参直接参考25题方法

--27、查询选修一门以上且不包括最高成绩学生的其余学生成绩

 --第一种
 select sid,cid,score from sc where sid not in (select sc.sid from sc  join (select cid,max(score) maxscore from sc group by cid ) m
 on m.maxscore=sc.score and   m.cid=sc.cid )  and sid in (select sid from sc  group by sid having count (sid)>1)
 --第二种
 select sid,cid,score from sc where sid not in
 ( select a.sid  from sc a left join sc b on a.cid=b.cid and a.score<b.score group by a.cid,a.sid,a.score having count(b.score)=0)
 and sid in (select sid from sc  group by sid having count (sid)>1)
 --若改成-选修一门以上不是最高成绩的其余成绩(还包括最高成绩的学生的其他科目成绩)--每科成绩非最高分的记录
 select cid,sid,score from sc a where  sid not  in  (
 select  top 3 b.sid  from sc b where a.cid=b.cid order by b.cid asc ,b.score desc )
 and sid in (select sid from sc  group by sid having count (sid)>1)

--28、查询每门课程被选修的学生数

 select cid,count(sid) 选修人数 from sc  group by cid 

--29、查询出只选修两门课程的学生学号和姓名

 select student.sid 学号,sname 姓名 from student join sc on student.sid=sc.sid group by student.sid,sname having count(student.sid)=2

--30、查询男生、女生人数

 select ssex, count (ssex) 人数 from student group by ssex

--31、查询名字中含有"风"字的学生信息

 select * from student where sname like '风%' or sname like '%风' 

--32、查询同名同性学生名单，并统计同名人数

 select a.sname,a.ssex,count(a.sname) 同名人数 from student a,student b
 where  a.sid!=b.sid and a.sname=b.sname and a.ssex=b.ssex  group by a.sname,a.ssex 

--33、查询1990年出生的学生名单

 select * from student where datename(yy,sage)=''

--34、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,cast(avg(score) as decimal(5,2)) avgscore from sc group by  cid order by avgscore desc,cid asc 

--35、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.sid,sname,avg(score) from student
 join sc on student.sid =sc.sid group by student.sid,sname having avg(score)>=85

--36、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,score from student join sc on student.sid=sc.sid
 join course on course.cid=sc.cid where cname='数学' and score <60 

--37、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.*,cid,score from student left join sc on student.sid=sc.sid 

--38、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.sid,course.cname,sc.score from student join sc on  student.sid=sc.sid join course on course.cid=sc.cid where score>70

--39、查询存在不及格的课程

 --第一种
 select distinct cid from sc where score<60--distinct确定唯一性
 --第二种
 select  cid from sc where score<60 group by cid--group by 确定唯一性

--40、查询课程编号为 01 且课程成绩在 80 分及以上的学生的学号和姓名

 select student.sid,student.sname from student join sc on student.sid=sc.sid where cid='' and score>=80

--41、求每门课程的学生人数
--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 cid,count(sid) 每门课程人数 from sc group by cid order by 每门课程人数 desc,cid asc 

--42、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

  select top 1 student.*,m.maxscore from student join (
 select sid,max(score) maxscore from teacher join course on course.tid=teacher.tid join sc on sc.cid=course.cid
 where tname='张三' group by sid ) m on  student.sid=m.sid order by m. maxscore desc

 --若改成：每个教师所教每门课程的最高成绩
 select m.tname,m.cid,sid,m.maxscore from sc join (
 select tname,sc.cid,max(score) maxscore from teacher join course on course.tid=teacher.tid join sc on sc.cid=course.cid
  group by tname,sc.cid) m on sc.cid=m.cid and sc.score=m.maxscore order by m.cid asc 

--43、成绩重复，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

 select student.*,m.score from student join (
  select a.cid,a.sid,a.score from sc a left join sc b on a.cid=b.cid and a.score<b.score  group by a.cid,a.sid,a.score having count(b.score)=0) m
  on student.sid=m.sid  join course on course.cid=m.cid join teacher on teacher.tid=course.tid where tname='张三'--m表内容也可以改成用rank函数做成排名，取排名第一即可

--44、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 a.sid,a.cid,a.score from sc a join sc b on a.sid=b.sid where  a.cid!=b.cid and a.score=b.score group by a.cid,a.sid,a.score

--45、查询选修了全部课程的学生信息

 select student.* from student join
 (select  sid from sc group by sid having count(sc.cid)= (select count(cid) from course)) m
 on student.sid=m.sid  

--46、查询各学生的年龄,只按照年份来算

  select student.*,datediff(yy,sage,getdate())  age from student 

--47、查询各学生的年龄,按照出生日期来算，当前月日 < 出生年月的月日则年龄减一

 --第一种
 select *,
 (case when convert(int,convert(varchar(10),getdate(),112))<convert (int,convert(varchar(10),year(getdate()))+substring(convert(varchar(10),sage,112),5,4))
 then datediff(yy,sage,getdate())-1 else datediff(yy,sage,getdate()) end )age--出生年月变成现在的年与月日 同现在的时间对比大小
 from student
 --第二种
 select *,(case when month(getdate())<month(sage) then  datediff(yy,sage,getdate())-1
 when month(getdate())=month(sage) and day(getdate())<day(sage) then datediff(yy,sage,getdate())-1
 else datediff(yy,sage,getdate()) end) age from student
 --第三种
 select * , (case when right(convert(varchar(10),getdate(),120),5) < right(convert(varchar(10),sage,120),5)
 then datediff(yy ,sage,getdate()) - 1 else datediff(yy ,sage, getdate()) end) age from student 

--48、查询本周过生日的学生

 --第一种
 select *,(case when datename(wk,convert(datetime,(convert(varchar(10),year(getdate()))+substring(convert(varchar(10),sage,112),5,4))))=
 datename(wk,getdate()) then 1 else 0 end) 生日提醒 from student
 --第二种
  select * from student where datediff(wk,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = 0

49、查询下周过生日的学生

 --第一种
  select *,(case when datename(wk,convert(datetime,(convert(varchar(10),year(getdate()))+substring(convert(varchar(10),sage,112),5,4))))=
 datename(wk,getdate())+1 then 1 else 0 end) 生日提醒 from student
 --第二种
  select * from student where datediff(wk,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = -1

--50、查询本月过生日的学生

  --第一种
 select * from student where month(getdate())=month(sage)
 --第二种
  select * from student where datediff(mm,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = 0

--51、查询下月过生日的学生

 --第一种
  select * from student where month(getdate())+1=month(sage)
  --第二种
 select * from student where datediff(mm,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = -1

【这是一条整合了全部51道题目及答案的代码】

 --1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数
 --第一种方法
 select student.*,m.score [01课程成绩], n.score [02课程成绩]from student
   join (select sid,score from sc where cid='') m on student.sid=m.sid
   join (select sid,score from sc where cid='')n  on student.sid=n.sid
 where m.score>n.score
 --第二种方法
 select student.*,m.[01课程成绩],m.[02课程成绩] from student,
 (select sid,sum(case when cid='' then score else null end) [01课程成绩] ,--若else后面是0,则结果会输出有01课程无02课程的学生,若改为null则不会输出只有一门课程的学生成绩
 sum(case when cid='' then score else null end)  [02课程成绩]from sc group by sid) m
 where student.sid=m.sid and m.[01课程成绩]>m.[02课程成绩]

   --2、查询同时存在"01"课程和"02"课程的情况

   select student.*,m.score [01课程成绩],n.score [02课程成绩] from student
   join (select sid ,score from sc where cid='') m on  student.sid=m.sid
 join  (select sid,score from sc where cid='') n on student.sid=n.sid

   --3、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 student.*,m.score,n.score  from (select sid,score from sc where cid='') m left join student on student.sid=m.sid
   left join (select sid,score from sc where cid='')n on student.sid=n.sid

   --4、 查询不存在" 01 "课程但存在" 02 "课程的情况
     select student.*,score from student join sc on student.sid=sc.sid where  cid='' and student.sid not in (select sid from sc where cid='')
 --5、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 student.sid,sname,cast (avg(score)  as decimal(5,2))平均成绩 from student join sc on student.sid=sc.sid
 group by student.sid,sname having avg(score)>=60
 --6、查询在sc表存在成绩的学生信息的SQL语句
 --第一种
 select distinct student.* from student join sc on student.sid=sc.sid
 --第二种
 select * from student where sid in (select sc.sid from sc )--IN()适合B表(括号内的表是B)比A表数据小的情况
 --第三种
 select * from student where exists (select sc.sid from sc where student.sid = sc.sid)--EXISTS()适合B表比A表数据大的情况
 --7、查询在sc表中不存在成绩的学生信息的SQL语句
 select * from student where sid  not in (select sc.sid from sc )
 --8、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(没成绩的显示为 null )
  select student.sid,sname,count(cid) 选课总数 ,sum(score ) 所有课程成绩的总和 from student
  left join sc on student.sid=sc.sid group by student.sid,sname
  --9、查询「李」姓老师的数量
  select count (tid) 李姓老师的数量  from teacher where tname like '李%'
  --10、查询学过「张三」老师授课的同学的信息
 select student.* from student join sc on student.sid=sc.sid
 join course on course.cid=sc.cid
 join teacher on teacher.tid=course.tid  where tname='张三'

 --11、查询没学过「张三」老师授课的同学的信息
 select  *from student where sid not in (select sc.sid from sc join course on course.cid=sc.cid
 join teacher on teacher.tid=course.tid  where tname='张三')

 --12、查询没有学全所有课程的同学的信息
 --第一种直接
 select student.*  from student left  join sc on student.sid=sc.sid group by student.sid,sname,sage,ssex
 having count(sc.cid)<(select count(course.cid) from course)
 ----第二种反向
 select student.* from student where student.sid not in
 (select sc.sid from sc group by sid having count(sc.cid)= (select count(cid) from course))
 --13、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 student.* from student  join sc on student.sid=sc.sid
  where cid in (select cid from sc where sid='') and student.sid!='' --in的意思是:等于其中的一个则输出该条相等数据，直至全部输出为止
 --14、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
 --(即所学课程数要一致,学的课程目录也要一致)
 select student.* from student join
 (select sid from sc  group  by sid having  count(cid)=(select count(cid) from sc where sid='' )) m--控制大范围下学生所学课程数=01同学的课程数3个,例如:学4门课程的同学,其中3门与01同学相同,也满足后续程序,但是与01同学就不是完全相同的.
  on student.sid=m.sid  join sc on student.sid=sc.sid
 where sc.cid in (select cid from sc where sid='') and student.sid!=''group by student.sid,sname,sage,ssex
 having count(sc.cid)=(select count(sc.cid) from sc where sid='')--控制与01同学所学课程目录相同的数也是3个
 --15、查询没学过"张三"老师讲授的任一门课程的学生姓名
 select * from student where student.sid not in
 (select sc.sid from sc join course on sc.cid=course.cid
 join teacher on teacher.tid=course.tid where  tname='张三' )
 --16、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
 --第一种
 select student.sid,sname,cast(avg(score) as decimal(5,2)) avgscore from student  join sc
 on student.sid=sc.sid where sc.score<60 group by student.sid,sname having count(sc.cid)>=2
 --第二种
 select student.sid,sname,cast(avg(score) as decimal(5,2)) avgscore from student join sc
 on student.sid=sc.sid group by student.sid,sname having sum(case when score<60 then 1 else 0 end)>=2
  --17、检索"01"课程分数小于60，按分数降序排列的学生信息
  select student.* from student join sc on student.sid=sc.sid where cid='' and score<60 order by score desc
  --18、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 student.sid,sname, sc.cid,sc.score,m.avgscore from student left join sc on student.sid=sc.sid
  left join (select sid,cast(avg(score) as decimal(5,2)) avgscore from sc group by sid ) m on sc.sid=m.sid
  order by m.avgscore desc
   --19、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 sc.cid,course.cname,max(sc.score) 最高分,min(sc.score) 最低分,cast(avg(sc.score) as decimal(5,2)) 平均分 ,
  count(sc.cid) 计数,
  sum(case when sc.score>=60 then 1 else 0 end )及格数, sum(case when sc.score>=70 and sc.score<80 then 1 else 0 end ) 中等数,
   sum(case when sc.score>=80 and sc.score<90 then 1 else 0 end ) 优良数,sum(case when sc.score>=90 then 1 else 0 end ) 优秀数,--这里的计数,及格数...优秀数是为了验证后续的及格率...优秀率是否正确的,完全按照题意需省去
  convert(varchar(10),cast((100*sum(case when score>=60 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 及格率,--分母count(sc.sid)要转化为浮点型数据，否则分子是整数，结果也为整数，舍小数位或者把分子转化为浮点型数据，结果也会为浮点型
  convert(varchar(10),cast((100*sum(case when sc.score>=70 and sc.score<80 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 中等率,
  convert(varchar(10),cast((100*sum(case when sc.score>=80 and sc.score<90 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 优良率,
  convert(varchar(10),cast((100*sum(case when sc.score>=90 then 1 else 0 end)  /cast(count(sc.cid) as float)) as decimal(5,2)))+'%' 优秀率
  from course join sc on course.cid=sc.cid group by sc.cid,course.cname

  --20、按各科成绩进行排序，并显示排名,score 重复时保留名次空缺
  --第一种
  select a.cid,a.sid,a.score,count(b.score) +1 排名 from sc a
  left join sc b on a.cid=b.cid and a.score<b.score  group by a.cid,a.sid,a.score order by a.cid asc,排名 asc
  ---select a.cid,a.sid,a.score,b.score from sc a left join sc b on a.cid=b.cid and a.score<b.score group by a.cid,a.sid,a.score,b.score 在课程相同的情况下，计算比自己分数大的有几个再加上1即为排名
  --例如课程3,学号1,99；2,80；3,80；4,90；6,34;7,98
  --则对应的比1,99大的为null排名1;比2,80大的为99,90,98排名4;比3,80大的99,90,98排名3;比4,90大的为99,98排名3；比6,34大的为99,90,98,80,80排名6；比7,98大的为99排名2

  --第二种(使用内置函数rank)
  select cid,sid,score,rank() over (partition  by cid order by score desc )  排名 from sc --partition by 分组 order by 排序
   --21、按各科成绩进行排序，并显示排名,score 重复时合并名次空缺
    --第一种
  select a.cid,a.sid,a.score,count(distinct b.score) +1 排名 from sc a
  left join sc b on a.cid=b.cid and a.score<b.score  group by a.cid,a.sid,a.score order by a.cid asc,排名 asc
   --第二种(使用内置函数dense_rank)
  select cid,sid,score,dense_rank() over (partition  by cid order by score desc )  排名 from sc

  --22、查询学生的总成绩，并进行排名，总分重复时合并名次空缺
  --第一种
 select a.sid,a.allscore 总成绩,count(distinct b.allscore)+1 排名 from ( select sid,sum(score) allscore from sc group by sid) a
 left join ( select sid,sum(score) allscore from sc group by sid) b on  a.allscore<b.allscore group by a.sid,a.allscore order by 排名 asc
 --a表与b表中已经进行完全相同的学生sid分组，在左连接时不应再让sid相等连接，否则左连接时b.allscore >a.allscore匹配为空值，每个学生的排名都为1，即在学号sid相等情况，不能再比较不同学生间的成绩排名,只能自我比较
 --而20/21题的第一种方法有cid左连接相等即在同一课程下，还可以比较不同学生sid间的成绩排名
 --第二种
 select sid,sum(score)  总成绩 ,dense_rank()over(order by sum(score) desc ) 排名 from sc group by sid
 --这里的成绩不是所有同学sid的成绩，有的同学没有成绩，因此我们可以将student与上述两种方法进行左连接--例如：
 select a.sid ,a.allscore 总成绩,count(distinct b.allscore)+1 排名 from
 (select student.sid ,isnull(sum(score),0) allscore from student left join sc  on student.sid=sc.sid group by student.sid ) a left join
 (select student.sid ,isnull(sum(score),0) allscore from student left join sc  on student.sid=sc.sid group by student.sid ) b on a.allscore<b.allscore
 group by a.sid,a.allscore order by 排名 asc

 select student.sid,isnull(sum(score),0) 总成绩,dense_rank()over(order by isnull(sum(score),0) desc )排名
  from student left join sc on student.sid=sc.sid group by student.sid

  --23、查询不同老师所教不同课程平均分从高到低显示
  select course.tid,tname,course.cid,cast(avg(score) as decimal(5,2)) 课程平均分 from course left join sc   on  sc.cid=course.cid
 left join teacher on teacher.tid=course.tid  group by course.tid ,tname,course.cid order by avg(score) desc

 --24、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
 --第一种
 select student.*,m.cid,m.成绩,m.排名 from student left join
 ( select  a.cid,a.sid,a.score 成绩,count(b.score)+1 排名 from sc a left join sc b on a.cid=b.cid and a.score<b.score
 group by a.cid,a.sid,a.score ) m on m.sid=student.sid where m.排名 between 2 and 3 order by m.cid asc,排名 asc--重复排名时保留名次空缺,把count 内改成distinct b.score 即为重复时合并空缺
 --第二种
 select student.*,m.cid,m.成绩,m.排名 from student left join
 (select cid,sid,score 成绩,rank()over(partition by cid order by score desc) 排名 from sc group by cid,sid,score) m
 on student.sid=m.sid  where m.排名 between 2 and 3 order by cid asc--rank 改为dense_rank 即为合并名次空缺

  --25、查询各科成绩前三名的记录
  --第一种
 select a.cid,a.sid,a.score from sc a left join sc b on a.cid=b.cid and a.score<b.score
 group by a.cid,a.sid,a.score having  count(b.score)<3 order  by a.cid asc ,a.score desc --成绩重复时保留名次空缺，输出排名为1-3的学生;也可以按照24题来做,排名改成between 1 and 3即可
 --第二种
 select cid,sid,score from sc a where  sid in  (
 select  top 3 b.sid  from sc b where a.cid=b.cid order by b.cid asc ,b.score desc )
 order by  cid asc,score  desc --这种top 型的只输出前3个学生，对于成绩重复的同一名次会缺失
 --第三种
 select * from
 (select *,row_number()over(partition by cid order by score desc) 排名 from sc) B
 where B.排名<4--同样只输出前3个学生，对于成绩重复的同一名次会缺失

 --26、查询各科成绩最高的记录
 select sc.cid, sc.sid,m.maxscore from sc  join (select cid,max(score) maxscore from sc group by cid ) m
 on m.maxscore=sc.score and   m.cid=sc.cid
 --其余方法可参直接参考25题方法

 --27、查询选修一门以上且不包括最高成绩学生的其余学生成绩
 --第一种
 select sid,cid,score from sc where sid not in (select sc.sid from sc  join (select cid,max(score) maxscore from sc group by cid ) m
 on m.maxscore=sc.score and   m.cid=sc.cid )  and sid in (select sid from sc  group by sid having count (sid)>1)
 --第二种
 select sid,cid,score from sc where sid not in
 ( select a.sid  from sc a left join sc b on a.cid=b.cid and a.score<b.score group by a.cid,a.sid,a.score having count(b.score)=0)
 and sid in (select sid from sc  group by sid having count (sid)>1)
 --若改成-选修一门以上不是最高成绩的其余成绩(还包括最高成绩的学生的其他科目成绩)--每科成绩非最高分的记录
 select cid,sid,score from sc a where  sid not  in  (
 select  top 3 b.sid  from sc b where a.cid=b.cid order by b.cid asc ,b.score desc )
 and sid in (select sid from sc  group by sid having count (sid)>1)

 --28、查询每门课程被选修的学生数
 select cid,count(sid) 选修人数 from sc  group by cid 

 --29、查询出只选修两门课程的学生学号和姓名
 select student.sid 学号,sname 姓名 from student join sc on student.sid=sc.sid group by student.sid,sname having count(student.sid)=2
 --30、查询男生、女生人数
 select ssex, count (ssex) 人数 from student group by ssex
 --31、查询名字中含有"风"字的学生信息
 select * from student where sname like '风%' or sname like '%风'
 --32、查询同名同性学生名单，并统计同名人数
 select a.sname,a.ssex,count(a.sname) 同名人数 from student a,student b
 where  a.sid!=b.sid and a.sname=b.sname and a.ssex=b.ssex  group by a.sname,a.ssex
 --33、查询1990年出生的学生名单
 select * from student where datename(yy,sage)=''
 --34、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,cast(avg(score) as decimal(5,2)) avgscore from sc group by  cid order by avgscore desc,cid asc
 --35、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.sid,sname,avg(score) from student
 join sc on student.sid =sc.sid group by student.sid,sname having avg(score)>=85
 --36、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,score from student join sc on student.sid=sc.sid
 join course on course.cid=sc.cid where cname='数学' and score <60
 --37、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.*,cid,score from student left join sc on student.sid=sc.sid
 --38、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.sid,course.cname,sc.score from student join sc on  student.sid=sc.sid join course on course.cid=sc.cid where score>70
 --39、查询存在不及格的课程
 select distinct cid from sc where score<60--distinct确定唯一性
 select  cid from sc where score<60 group by cid--group by 确定唯一性
 --40、查询课程编号为 01 且课程成绩在 80 分及以上的学生的学号和姓名
 select student.sid,student.sname from student join sc on student.sid=sc.sid where cid='' and score>=80
 --41、求每门课程的学生人数
 --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 cid,count(sid) 每门课程人数 from sc group by cid order by 每门课程人数 desc,cid asc
 --42、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
  select top 1 student.*,m.maxscore from student join (
 select sid,max(score) maxscore from teacher join course on course.tid=teacher.tid join sc on sc.cid=course.cid
 where tname='张三' group by sid ) m on  student.sid=m.sid order by m. maxscore desc

 --若改成：每个教师所教每门课程的最高成绩
 select m.tname,m.cid,sid,m.maxscore from sc join (
 select tname,sc.cid,max(score) maxscore from teacher join course on course.tid=teacher.tid join sc on sc.cid=course.cid
  group by tname,sc.cid) m on sc.cid=m.cid and sc.score=m.maxscore order by m.cid asc 

  --43、成绩重复，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
  select student.*,m.score from student join (
  select a.cid,a.sid,a.score from sc a left join sc b on a.cid=b.cid and a.score<b.score  group by a.cid,a.sid,a.score having count(b.score)=0) m
  on student.sid=m.sid  join course on course.cid=m.cid join teacher on teacher.tid=course.tid where tname='张三'--m表内容也可以改成用rank函数做成排名，取排名第一即可

  --44、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 a.sid,a.cid,a.score from sc a join sc b on a.sid=b.sid where  a.cid!=b.cid and a.score=b.score group by a.cid,a.sid,a.score

 --45、查询选修了全部课程的学生信息
 select student.* from student join
 (select  sid from sc group by sid having count(sc.cid)= (select count(cid) from course)) m
 on student.sid=m.sid
  --46、查询各学生的年龄,只按照年份来算
  select student.*,datediff(yy,sage,getdate())  age from student
 --47、查询各学生的年龄,按照出生日期来算，当前月日 < 出生年月的月日则年龄减一
 --第一种
 select *,
 (case when convert(int,convert(varchar(10),getdate(),112))<convert (int,convert(varchar(10),year(getdate()))+substring(convert(varchar(10),sage,112),5,4))
 then datediff(yy,sage,getdate())-1 else datediff(yy,sage,getdate()) end )age--出生年月变成现在的年与月日 同现在的时间对比大小
 from student
 --第二种
 select *,(case when month(getdate())<month(sage) then  datediff(yy,sage,getdate())-1
 when month(getdate())=month(sage) and day(getdate())<day(sage) then datediff(yy,sage,getdate())-1
 else datediff(yy,sage,getdate()) end) age from student
 --第三种
 select * , (case when right(convert(varchar(10),getdate(),120),5) < right(convert(varchar(10),sage,120),5)
 then datediff(yy ,sage,getdate()) - 1 else datediff(yy ,sage, getdate()) end) age from student
 --48、查询本周过生日的学生
 --第一种
 select *,(case when datename(wk,convert(datetime,(convert(varchar(10),year(getdate()))+substring(convert(varchar(10),sage,112),5,4))))=
 datename(wk,getdate()) then 1 else 0 end) 生日提醒 from student
 --第二种
  select * from student where datediff(wk,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = 0
  --49、查询下周过生日的学生
  --第一种
  select *,(case when datename(wk,convert(datetime,(convert(varchar(10),year(getdate()))+substring(convert(varchar(10),sage,112),5,4))))=
 datename(wk,getdate())+1 then 1 else 0 end) 生日提醒 from student
 --第二种
  select * from student where datediff(wk,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = -1
  --50、查询本月过生日的学生
  --第一种
 select * from student where month(getdate())=month(sage)
 --第二种
  select * from student where datediff(mm,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = 0

  --51、查询下月过生日的学生
  --第一种
  select * from student where month(getdate())+1=month(sage)
  --第二种
 select * from student where datediff(mm,datename(yy,getdate()) + right(convert(varchar(10),sage,120),6),getdate()) = -1