SQL语句学习题库

博主： zmx
发布时间：2021 年 05 月 30 日
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分类： MySQL

# SQL语句学习题库

# 引言

摘抄自互联网，稍有修改，最近自动生成的太多了，所以感觉应该练习一下，还有昂，如有侵权，请联系站长，予以删除处理。

## MySQL版本：MySQL8.0

## 测试表格：

Student (SId,Sname,Sage,Ssex)

Course (CId,Cname,TId)

Teacher (TId,Tname)

SC (SId,CId,score)

## 建数据库，建表格：

### Student：

```sql
CREATE DATABASE practise;
USE practise;
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' , '1989-12-20' , '女');
INSERT INTO Student VALUES('10' , '李四' , '1989-12-25' , '女');
INSERT INTO Student VALUES('11' , '李四' , '1991-12-30' , '女');
INSERT INTO Student VALUES('12' , '赵六' , '1992-01-01' , '女');
INSERT INTO Student VALUES('13' , '孙七' , '1991-01-01' , '女');
```

### Course：

```sql
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');
```

### Teacher：

```sql
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');
```

### SC：

```sql
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);
```

## 练习题：

### 1. 查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

```sql
select * 
from (select * from SC where CId = '01') A, (select * from SC where CId = '02') B 
where A.SId = B.SId and A.Score > B.Score;
```

### 2.查询同时存在" 01 "课程和" 02 "课程的情况

```mysql
select * 
from (select * from SC where CId = '01') A, (select * from SC where CId = '02') B 
where A.SId = B.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 )

```sql
select *
from (select SId ,score from SC where CId = '01') A 
left join (select SId ,score from SC where CId = '02') B
on A.SId = B.SId
```

### 4. 查询不存在" 01 "课程但存在" 02 "课程的情况

```sql
select * from SC 
where CId = '02' and SId not in (select SId from SC where CId = '01');
```

### 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

```sql
select S.SId,S.Sname,T.avgscore from Student S 
inner join (select SC.SId ,AVG(SC.Score) avgscore from SC 
group by SC.SId having AVG(SC.Score)>60) T on S.SId = T.SId;
```

### 6. 查询在 SC 表存在成绩的学生信息

```sql
select distinct S.* 
from Student S,SC 
where S.SId = SC.SId;
```

### 7. 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 )

```sql
select S.SId,S.Sname,T.count,T.sum 
from Student S ,(select SId,COUNT(CId) count, SUM(score) sum from SC group by SId) T 
on S.SId = T.SId;
```

### 8.查有成绩的学生信息

```sql
select distinct S.* 
from Student S,(select SId from SC where score is not null) T 
where S.SId = T.SId;
```

### 9.查询「李」姓老师的数量.

```sql
select count(TId) from Teacher 
where Tname like '李%';
```

### 10.查询学过「张三」老师授课的同学的信息

```sql
select S.* from Student S,Course C,Teacher T,SC 
where T.Tname ='张三' and T.TId = C.TId and C.CId = SC.CId and SC.SId = S.SId;
```

### 11.查询没有学全所有课程的同学的信息

```sql
select S.* 
from Student S,(select SId,count(CId) from SC group by SId having count(CId) < 3) T 
where S.SId = T.SId;
```

### 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

```sql
select distinct S.* from Student S,SC 
where S.SId = SC.SId and SC.CId in (select CId from SC where SId ='01');
```

### 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

```sql
select * from Student 
where SId in(select SId from SC where CId in (select CId from SC where SId = '01') 
and SId <> '01' group by SId having count(CId)=3);
```

### 14.查询没学过"张三"老师讲授的任一门课程的学生姓名

```sql
select Student.Sname from Student 
where Student.SId not in(select SC.SId from Teacher T,Course C,SC where T.Tname ='张三' 
and T.TId = C.TId and C.CId = SC.CId);
```

### 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

```sql
select S.Sname,S.SId,T.Avgscore from Student S,(select SID, AVG(Score) Avgscore from SC 
where score < 60 group by SId having count(CId) >= 2) T 
where S.SId = T.SId;
```

### 16.检索" 01 "课程分数小于 60，按分数降序排列的学生信息

```sql
select S.* from Student S,
(select SId  from SC where CId = '01' and score < 60 order by score desc) T 
where S.SId = T.SId;
```

### 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

```sql
select SC.*,T.Avgscore from SC ,(select SId,AVG(score) avgscore from sc group by SId) T
where SC.SId = T. SId order by T.Avgscore desc;
```

### 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

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

```sql
select CId,MAX(score),MIN(score),AVG(score),
SUM(case when score >= 60 then 1 else 0 end)/count(SId) 及格率,
SUM(case when score >= 70 and score < 80 then 1 else 0 end)/count(SId) 中等率,
SUM(case when score >= 80 and score < 90 then 1 else 0 end)/count(SId) 优良率,
SUM(case when score >= 90 then 1 else 0 end)/count(SId) 优秀率 
from SC group by CId order by count(SId) desc,CId asc;
```

### 19.按各科成绩进行排序，并显示排名， Score 重复时保留名次空缺

```sql
select *,RANK()over(order by score desc)排名 from SC;
```

### 20. 按各科成绩进行排序，并显示排名， Score 重复时合并名次

```sql
select *,DENSE_RANK()over(order by score desc)排名 from SC;
```

### 21.查询学生的总成绩，并进行排名，总分重复时保留名次空缺

```sql
select *,RANK()over(order by 总成绩 desc)排名 
from (select SId,SUM(score)总成绩 from SC group by SId)A;
```

### 22.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

```sql
select *,dense_rank()over(order by 总成绩 desc)排名 
from(select SId,SUM(score)总成绩 from SC group by SId)A;
```

### 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

```sql
select C.Cname, T.* from Course C 
left join (select SC.CId,CONCAT(SUM(case when SC.score <= 60 then 1 else 0 end)/count(SC.SId)*100,'%') '[0-60]',
CONCAT(SUM(case when SC.score >60 and SC.score <= 70 then 1 else 0 end)/count(SC.SId)*100,'%') '[60-70]',
CONCAT(SUM(case when SC.score >70 and SC.score <= 85 then 1 else 0 end)/count(SC.SId)*100,'%') '[70-85]',
CONCAT(SUM(case when SC.score >85 and SC.score <= 100 then 1 else 0 end)/count(SC.SId)*100,'%') '[85-100]', 
from SC group by SC.CId) T on C.CId = T.CId;
```

### 24.查询各科成绩前三名的记录

```sql
select * from(select *,rank()over (partition by CID order by score desc) 排名 from SC)B 
where B.排名<=3;
```

### 25.查询每门课程被选修的学生数

```sql
select CId,count(SId) from SC group by CId;
```

### 26.查询出只选修两门课程的学生学号和姓名

```sql
select S.SId,S.Sname from Student S 
where S.SId in (select SC.SId from SC group by SC.SId having count(SC.CId) = 2);
```

### 27.查询男生、女生人数

```sql
select Student.Ssex ,count(*) 人数 
from student group by Student.Ssex;
```

### 28.查询名字中含有「风」字的学生信息

```sql
select * from Student 
where Sname like '%风%';
```

### 29.查询同名同性学生名单，并统计同名人数

```sql
select * from student 
left join (select Sname,Ssex,count(*)人数 from Student group by Sname,Ssex) T
on Student.Sname =T.Sname and Student.Ssex=T.Ssex where T.人数>1;
```

### 30.查询 1990 年出生的学生名

```sql
select * from Student 
where year(Sage)=1990;
```

### 31.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

```sql
select CId,AVG(score) 平均成绩 from SC 
group by CId order by 平均成绩 desc,CId asc;
```

### 32.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

```sql
select S.SId,S.Sname,T.平均成绩 from Student S,
(select SC.SId,AVG(SC.score) 平均成绩 from SC group by SC.SId having 平均成绩 > 85) T 
where S.SId = T.SId;
```

### 33.查询课程名称为「数学」，且分数低于 60 的学生姓名和分数

```sql
select S.Sname,SC.score from Student S, SC, Course C 
where S.SId = SC.SId and SC.score < 60 and SC.CId = C.CId and C.Cname = '数学';
```

### 34.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）

```sql
select S.SId,sc.CId,SC.score 
from Student S left join SC on S.SId = SC.SId;
```

### 35.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

```sql
select S.Sname,C.Cname,SC.score from Student S,Course C,SC 
where SC.score>=70 and S.SId = SC.SId and SC.CId=C.CId;
```

### 36.查询不及格的课程

```sql
select DISTINCT CId from SC where score <60;
```

### 37.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

```sql
select SC.SId,S.Sname from Student S,SC 
where SC.CId='01'and S.SId=SC.SId and SC.score>80;
```

### 38.求每门课程的学生人数

```sql
select CId,count(SId) 人数 from SC group by CId;
```

### 39.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

```sql
select S.*,SC.score from Student S,Course C,Teacher T,SC 
where C.CId=SC.CId and C.TId=T.TId and T.Tname='张三' 
and S.SId =SC.SId order by SC.score desc limit 1;
```

### 40.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

```sql
select * from (select SC.SId, dense_rank()over(partition by SC.CId order by SC.score desc) 排名 from SC 
where SC.CId in(select C.CId from Course C, Teacher T where C.TId = T.TId and T.Tname = '张三')) T1
where T1.排名 = 1;
```

### 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

```sql
select * from SC T1 
where exists(select * from SC T2 where T1.SId=T2.SId and T1.CId!=T2.CId and T1.score =T2.score );
```

### 42.查询每门功成绩最好的前两名

```sql
select S.* from Student S 
inner join(select SC.SId, row_number()over (partition by SC.CId order by SC.score) 排名 from SC)T 
where T.排名 < 3 and S.SId = T.SId;
```

### 43.统计每门课程的学生选修人数（超过 5 人的课程才统计）

```sql
select CId, count(SId) from SC group by CId having count(SId) > 5;
```

### 44.检索至少选修两门课程的学生学号

```sql
select SId from SC group by SId having count(CId) >= 2;
```

### 45.查询选修了全部课程的学生信息

```sql
select S.* from Student S 
where S.SId in (select SId from SC group by SId having count(CId) = 3);
```

### 46.查询各学生的年龄，只按年份来算

```sql
select SId,Sname,TIMESTAMPDIFF(YEAR,Sage,now())年龄 from student;
```

### 47.按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一

```sql
select SId,Sname,(DATE_FORMAT(NOW(),'%Y')-DATE_FORMAT(Sage,'%Y') 
-(case when DATE_FORMAT(NOW(),'%m%d')>DATE_FORMAT(Sage,'%m%d') then 0 else 1 end)) 年龄 
from student;
```

### 48.查询本周过生日的学生

```sql
select * from Student where WEEK(Sage)=WEEK(now());
```

### 49. 查询下周过生日的学生

```sql
select * from Student where WEEK(Sage)=WEEK(now()) + 1;
```

### 50.查询本月过生日的学生

```sql
select * from Student where month(Sage)=month(now());
```

### 51. 查询下月过生日的学生

```sql
select * from Student where month(Sage)=month(now()) + 1;
```

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1 条评论

zmx
2021-5-30 22:46:58

答案有可能不对昂，要相信自己。❀

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SQL语句学习题库

zmx • 2021 年 05 月 30 日

# SQL语句学习题库

# 引言

摘抄自互联网，稍有修改，最近自动生成的太多了，所以感觉应该练习一下，还有昂，如有侵权，请联系站长，予以删除处理。

## MySQL版本：MySQL8.0

## 测试表格：

Student (SId,Sname,Sage,Ssex)

Course (CId,Cname,TId)

Teacher (TId,Tname)

SC (SId,CId,score)

## 建数据库，建表格：

### Student：

### Course：

### Teacher：

### SC：

## 练习题：

### 1. 查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

```sql
select * 
from (select * from SC where CId = '01') A, (select * from SC where CId = '02') B 
where A.SId = B.SId and A.Score > B.Score;
```

### 2.查询同时存在" 01 "课程和" 02 "课程的情况

```mysql
select * 
from (select * from SC where CId = '01') A, (select * from SC where CId = '02') B 
where A.SId = B.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 )

```sql
select *
from (select SId ,score from SC where CId = '01') A 
left join (select SId ,score from SC where CId = '02') B
on A.SId = B.SId
```

### 4. 查询不存在" 01 "课程但存在" 02 "课程的情况

```sql
select * from SC 
where CId = '02' and SId not in (select SId from SC where CId = '01');
```

### 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

```sql
select S.SId,S.Sname,T.avgscore from Student S 
inner join (select SC.SId ,AVG(SC.Score) avgscore from SC 
group by SC.SId having AVG(SC.Score)>60) T on S.SId = T.SId;
```

### 6. 查询在 SC 表存在成绩的学生信息

```sql
select distinct S.* 
from Student S,SC 
where S.SId = SC.SId;
```

### 7. 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 )

```sql
select S.SId,S.Sname,T.count,T.sum 
from Student S ,(select SId,COUNT(CId) count, SUM(score) sum from SC group by SId) T 
on S.SId = T.SId;
```

### 8.查有成绩的学生信息

```sql
select distinct S.* 
from Student S,(select SId from SC where score is not null) T 
where S.SId = T.SId;
```

### 9.查询「李」姓老师的数量.

```sql
select count(TId) from Teacher 
where Tname like '李%';
```

### 10.查询学过「张三」老师授课的同学的信息

```sql
select S.* from Student S,Course C,Teacher T,SC 
where T.Tname ='张三' and T.TId = C.TId and C.CId = SC.CId and SC.SId = S.SId;
```

### 11.查询没有学全所有课程的同学的信息

```sql
select S.* 
from Student S,(select SId,count(CId) from SC group by SId having count(CId) < 3) T 
where S.SId = T.SId;
```

### 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

```sql
select distinct S.* from Student S,SC 
where S.SId = SC.SId and SC.CId in (select CId from SC where SId ='01');
```

### 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

```sql
select * from Student 
where SId in(select SId from SC where CId in (select CId from SC where SId = '01') 
and SId <> '01' group by SId having count(CId)=3);
```

### 14.查询没学过"张三"老师讲授的任一门课程的学生姓名

```sql
select Student.Sname from Student 
where Student.SId not in(select SC.SId from Teacher T,Course C,SC where T.Tname ='张三' 
and T.TId = C.TId and C.CId = SC.CId);
```

### 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

```sql
select S.Sname,S.SId,T.Avgscore from Student S,(select SID, AVG(Score) Avgscore from SC 
where score < 60 group by SId having count(CId) >= 2) T 
where S.SId = T.SId;
```

### 16.检索" 01 "课程分数小于 60，按分数降序排列的学生信息

```sql
select S.* from Student S,
(select SId  from SC where CId = '01' and score < 60 order by score desc) T 
where S.SId = T.SId;
```

### 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

```sql
select SC.*,T.Avgscore from SC ,(select SId,AVG(score) avgscore from sc group by SId) T
where SC.SId = T. SId order by T.Avgscore desc;
```

### 18.查询各科成绩最高分、最低分和平均分：

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

### 19.按各科成绩进行排序，并显示排名， Score 重复时保留名次空缺

```sql
select *,RANK()over(order by score desc)排名 from SC;
```

### 20. 按各科成绩进行排序，并显示排名， Score 重复时合并名次

```sql
select *,DENSE_RANK()over(order by score desc)排名 from SC;
```

### 21.查询学生的总成绩，并进行排名，总分重复时保留名次空缺

```sql
select *,RANK()over(order by 总成绩 desc)排名 
from (select SId,SUM(score)总成绩 from SC group by SId)A;
```

### 22.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

```sql
select *,dense_rank()over(order by 总成绩 desc)排名 
from(select SId,SUM(score)总成绩 from SC group by SId)A;
```

### 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

### 24.查询各科成绩前三名的记录

```sql
select * from(select *,rank()over (partition by CID order by score desc) 排名 from SC)B 
where B.排名<=3;
```

### 25.查询每门课程被选修的学生数

```sql
select CId,count(SId) from SC group by CId;
```

### 26.查询出只选修两门课程的学生学号和姓名

```sql
select S.SId,S.Sname from Student S 
where S.SId in (select SC.SId from SC group by SC.SId having count(SC.CId) = 2);
```

### 27.查询男生、女生人数

```sql
select Student.Ssex ,count(*) 人数 
from student group by Student.Ssex;
```

### 28.查询名字中含有「风」字的学生信息

```sql
select * from Student 
where Sname like '%风%';
```

### 29.查询同名同性学生名单，并统计同名人数

```sql
select * from student 
left join (select Sname,Ssex,count(*)人数 from Student group by Sname,Ssex) T
on Student.Sname =T.Sname and Student.Ssex=T.Ssex where T.人数>1;
```

### 30.查询 1990 年出生的学生名

```sql
select * from Student 
where year(Sage)=1990;
```

### 31.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

```sql
select CId,AVG(score) 平均成绩 from SC 
group by CId order by 平均成绩 desc,CId asc;
```

### 32.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

```sql
select S.SId,S.Sname,T.平均成绩 from Student S,
(select SC.SId,AVG(SC.score) 平均成绩 from SC group by SC.SId having 平均成绩 > 85) T 
where S.SId = T.SId;
```

### 33.查询课程名称为「数学」，且分数低于 60 的学生姓名和分数

```sql
select S.Sname,SC.score from Student S, SC, Course C 
where S.SId = SC.SId and SC.score < 60 and SC.CId = C.CId and C.Cname = '数学';
```

### 34.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）

```sql
select S.SId,sc.CId,SC.score 
from Student S left join SC on S.SId = SC.SId;
```

### 35.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

```sql
select S.Sname,C.Cname,SC.score from Student S,Course C,SC 
where SC.score>=70 and S.SId = SC.SId and SC.CId=C.CId;
```

### 36.查询不及格的课程

```sql
select DISTINCT CId from SC where score <60;
```

### 37.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

```sql
select SC.SId,S.Sname from Student S,SC 
where SC.CId='01'and S.SId=SC.SId and SC.score>80;
```

### 38.求每门课程的学生人数

```sql
select CId,count(SId) 人数 from SC group by CId;
```

### 39.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

```sql
select S.*,SC.score from Student S,Course C,Teacher T,SC 
where C.CId=SC.CId and C.TId=T.TId and T.Tname='张三' 
and S.SId =SC.SId order by SC.score desc limit 1;
```

### 40.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

### 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

```sql
select * from SC T1 
where exists(select * from SC T2 where T1.SId=T2.SId and T1.CId!=T2.CId and T1.score =T2.score );
```

### 42.查询每门功成绩最好的前两名

```sql
select S.* from Student S 
inner join(select SC.SId, row_number()over (partition by SC.CId order by SC.score) 排名 from SC)T 
where T.排名 < 3 and S.SId = T.SId;
```

### 43.统计每门课程的学生选修人数（超过 5 人的课程才统计）

```sql
select CId, count(SId) from SC group by CId having count(SId) > 5;
```

### 44.检索至少选修两门课程的学生学号

```sql
select SId from SC group by SId having count(CId) >= 2;
```

### 45.查询选修了全部课程的学生信息

```sql
select S.* from Student S 
where S.SId in (select SId from SC group by SId having count(CId) = 3);
```

### 46.查询各学生的年龄，只按年份来算

```sql
select SId,Sname,TIMESTAMPDIFF(YEAR,Sage,now())年龄 from student;
```

### 47.按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一

```sql
select SId,Sname,(DATE_FORMAT(NOW(),'%Y')-DATE_FORMAT(Sage,'%Y') 
-(case when DATE_FORMAT(NOW(),'%m%d')>DATE_FORMAT(Sage,'%m%d') then 0 else 1 end)) 年龄 
from student;
```

### 48.查询本周过生日的学生

```sql
select * from Student where WEEK(Sage)=WEEK(now());
```

### 49. 查询下周过生日的学生

```sql
select * from Student where WEEK(Sage)=WEEK(now()) + 1;
```

### 50.查询本月过生日的学生

```sql
select * from Student where month(Sage)=month(now());
```

### 51. 查询下月过生日的学生

```sql
select * from Student where month(Sage)=month(now()) + 1;
```

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