sql基础知识

查询
select id,name,class,score from class
Select * from class
查不一样值
select DISTINCT class from class
前5条，mysql不支持top写法
Select * from class LIMIT 5
第5行起的后面5条数据 offset(第几条开始)
Select * from class limit 5 offset 5
升序
Select * from class order by score
降序
Select * from class order by score desc
数据过滤
Select * from class where class = 1
Select * from class where class != 1
Select * from class where class BETWEEN 1 and 2
Select * from class where class=1 or class=2
Select * from class where class=2 and name='lm'
通配符 Select * from class where name like '小%'
汇总
Select avg(score) from class
Select count(score) from class
Select max(score) from class
Select min(score) from class
Select sum(score) from class
分组求和
Select sum(score),class from class group by class
表链接
Select name,a.class,info from class a,grade b where a.class=b.class
内连
Select name,a.class,info from class a inner join grade b on a.class =
b.class
A的外链接
Select name,a.class,info from class a left outer join grade b on a.class = b.class
结连 UNION all 全部的结连，没有all的时候至关于多行DISTINCT
select class from class UNION all select class from grade
插入
insert into grade values(null,5,'五班')
insert into grade(class,info)values(5,'五班')
Insert into grade(class,info) select class,info from grade
更新
update class set class=5 where class=4
删除
delete from class where class=5
建立表
create table demo(id char(40) null, code char(49) not null)
删除表
drop table demo