MySql经典语句

1.三个函数： LENGTH  SUBSTR   LOCATE

select * from hbl_product where LENGTH(SUBSTR(WebSiteShopUrl,LOCATE('=',WebSiteShopUrl)+1))=4    order by WebSiteShopUrl   desc

2.序号 两种方法

第一种方法：
select   (@i:=@i+)   as   i,table_name.*   from   table_name,(select   @i:=)   as   it
第二种方法：

set @rownum=;
select @rownum:=@rownum+ as rownum, t.username from auth_user t limit ,;

3.ifnull 如果为null，则为0

ifnull(hci.Count,0)