1、SQL统计某字段的出现次数

比如统计某个表中，姓名出现的次数：
select name,count(*) from biao group by name having count(*) > 2

关键是用分组：group by，且经常和聚合函数一起使用

比如：统计用户表中的匿名字段的出现次数

SELECT  a.user_anon_name as anon_name  , COUNT(a.user_anon_name) as num FROM xm_user a GROUP BY a.user_anon_name

标准写法：

select 类别, sum(数量) as 数量之和 from A

group by 类别

having sum(数量) > 18

 2、SQL中如何用一个表的列更新另一个表的列

update T2 
set 要更新的字段 = T1.对应的字段　--可以输入多个用逗号分开
from T1 
where T2.ID = T1.ID

3、SQL 多字段排序
select * from (select * from tablename order by last_time desc) as t order by t.id desc