mysql 高級sql

插入新的列

alter table  tablename add colmun name type

求個數

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個字段

select * from tablename limit N

sql裏面selct  last 能夠返回最後一個，可是我沒找到，因此查到能夠這麼實現

select columnname1 from tablename order by columnname2 limit 1

求最大、最小、總值

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

sql group by 進行分組

select * from userinfo group by username order by id