MySQL--单表查询

创建数据表grade：

CREATE TABLE grade(
    id INT NOT NULL,
    sex CHAR(1),
    firstname VARCHAR(20) NOT NULL,
    lastname VARCHAR(20) NOT NULL,
    english FLOAT,
    math FLOAT,
    chinese FLOAT
);

向数据表grade中插入几条数据：

INSERT INTO grade
VALUES (1,'m','John','Smith',88.0,85.0,82.0),
(2,'f','Adam','Smith',76.0,78.0,90.0),
(3,'m','Allen','William',88.0,92.0,95.0),
(4,'m','George','William',62.0,58.0,72.0),
(5,'f','Alice','Davis',89.0,94.0,98.0),
(6,'m','Kevin','Miller',77.0,88.0,99.0),
(7,'f','Helen','Davis',79.0,83.0,91.0),
(8,'m','Andrew','Johnson',81.0,86.0,88.0);  

1、查询所有字段

select * from grade;

2、查询grade表中的id,firstname,lastname字段

select id,firstname,lastname from grade;

3、查询grade表中id大于4的学生姓名

 select id,firstname,lastname from grade where id > 4;

4、查询grade表中女生的记录

 select * from grade where sex = 'f';

5、查询grade表中id值为2,4,6的学生记录

 select * from grade where id = 2 or id = 4 or id = 6;

6、查询grade表中math成绩在85到94之间的记录

select * from grade where math between 85 and 94;

7、查询grade表中firstname以A开头的记录

select * from grade where firstname like 'A%';

8、查询grade表中firstname以A开头以e结尾的记录

select * from grade where firstname like 'A%e';

9、查询grade表中firstname包含l的记录

select * from grade where firstname like '%l%';

10、查询grade表中firstname以A开头后面有4个字符的记录

 select * from grade where firstname like 'A____';

11、查询grade表中english在80到90之间的记录

select * from grade where english between 80 and 90;

12、查询grade表中math大于90 或者 chinese大于90的记录

select * from grade where math > 90 or chinese > 90;

13、查询grade表中id不是1、3、5、7的记录

select * from grade where  id not in (1,3,5,7);

14、查询grade表中的性别有哪些

select distinct sex from grade;

15、查询grade表中的lastname有哪几种

select distinct lastname from grade;

16、求出表中所有记录的条数

 select count(*) from grade;

17、求出表中英语成绩是80的记录的条数

select count(*) from grade where english = 80;

18、计算所有学生的数学成绩的和

select sum(math) from grade;

19、计算女生的数学成绩的和

select sum(math) from grade where sex = 'f';

20、计算英语成绩平均分

 select avg(english) from grade;

21、计算男生的英语成绩平均分

select avg(english) from grade where sex = 'm';

22、求出数学成绩的最高分

 select max(math) from grade;

23、求出男生中的数学最高分

select max(math) from grade where sex = 'm';

24、按照math成绩的升序进行排列

select * from grade order by math;

25、按照sex字段的升序和chinese字段的降序排列

 select * from grade order by sex,chinese desc;

26、查询数据表grade中的记录，按照sex字段进行分组

 select sex from grade group by sex;

27、将grade表按照lastname字段值分组，并计算每个分组中的学生数

select lastname,count(*) from grade group by lastname;

28、对grade表按照lastname字段分组，查出math字段和小于100的组

 select lastname from grade group by lastname having sum(math) <100;

29、查询grade表中的第3到第6条记录

select * from grade order by id limit 2,4;

30、查询grade表中男生平均成绩(三科)大于85的记录

 select * from grade where sex = 'm' and (english + math + chinese)/3 > 85;