mysql 高级sql

插入新的列

alter table  tablename add colmun name type

求个数

select count(columnname) from tablename

求均值

select avg(columnname)  from tablename

求最小值

select min(columnname)  from tablename

查前n个字段

select * from tablename limit N

sql里面selct  last 可以返回最后一个，但是我没找到，所以查到可以这么实现

select columnname1 from tablename order by columnname2 limit 1

求最大、最小、总值

select max(columnname)/min(columnname)/sum(columnname) from tablename

sql group by 进行分组

select * from userinfo group by username order by id