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MySQL数据库学习day02

waaagh 2022-02-01 阅读 46

day02

昨天学习了一些简单的MySQL语句,今天呢,继续

1、排序:order by

升序:select * from 表名 order by 字段1 asc,字段2 asc;(先按字段1排序,若字段1相同则按字段2…)

降序:select * from 表名 order by 字段1 desc,字段2 desc;

例:select * from students order by age desc,studentNo desc;

2、聚合函数 count(*),max(*),min(*),sum(*),avg(*)等

count(*):求某类字段的个数

select count(*) from students where sex='女';

max(*):求某类字段的最大值

select max(age) from students where name like '孙%';

min(*):求某类字段的最小值

sum(*):求某类字段的总和

select sum(age) from students where sex='男';

avg(*):求某类字段的平均值

select avg (age) from students where class='1班';

3、分组:group by

select 字段1,字段2,聚合函数 from 表名 group by 字段1,字段2

select sex,count(*) from students group by sex ;

4、分页:limit 起始,个数(行数从0开始)

select * from students limit 0,3;

5、连接查询——等值连接 查询的结果为两个表匹配到的数据

select * from students, course where students.studentNo=course.studentNo

6、连接查询——内连接 

select * from students as stu inner join courses as c on stu.studentNo=c.studentNo;

SELECT stu.name,sc.courseNo,sc.score FROM students as stu INNER JOIN scores as sc on stu.studentsNo=sc.studentno WHERE stu.name='王昭君';

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