MySQL第二天

Posted 2020-11-04

查询

模糊查询(like)

范围查询

非连续 select * from students where id in(1,3,8);

连续 select * from students where id between 3 and 8 and gender=1;

聚合

快速得到统计数据

1. count(*)
2. sum()
3. max()
4. min()
5. avg()

示例： select count(*) from students
select sum(id) from students where isDelete=0 ;
子查询：select * from students where id=(select min(id) from students where idDelete=0);

分组(group by)

select gender as xb,count(*) as rs from students group by gender;
分组后筛选
select gender,count(*) from students group by gender having gender=1;
having&where:

where是对原始数据的筛选，针对from

having是对分组后结果进行筛选，针对group by

排序(order by)

默认升序

select * from 表名 order by 列1 asc|desc,列2 asc|desc,……    

分页

select * from 表名 limit start,count   

• 从start开始(0)，获取count条数据

• 第n页，显示m条数据
  select * from students<br/>where isdelete=0<br/>limit (n-1)*m,m;

  总结

  select (distinct) * 
  from 表名 
  where .... 
  group by ... 
  having ... 
  order by ... 
  limit start,count;  

  连接查询

  表与表之间建立关系

  select students.name,subjects.title,scores.score
  from scores
  inner join students on scores.stuid=students.id
  inner join subjects on scores.subid=subjects.id;

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通用模板

select distinct 列 *
from 表名1 
inner|left|right join 表名2 on 表之间的关系
group by ……having……
order by ……asc|desc
limit start,count

5/11/2018 6:35:33 PM