1，改变数据库状态

增：insert

删：delete

改：update

2不改变数据库状态

查：select ..输出..from..获取数据... where..过滤...group by ...分组...having...过滤...order by...排序....limit...限定个数...

执行顺序1from，2 where 3 group by 4 select 5 having 6 order by 7 limit