sql3

一 获取重复的数据的值
select user_name from users
group by user_name
having(count(1)>1)

二 获取重复数据的记录
select * from users where user_name in
(
select user_name from users
group by user_name
having(count(1)>1)
)

三 获取多余的重复数据
select * from users where user_name not in
(
select max(user_name) from users
group by user_name
having(count(1)>1)
)

 

INSERT INTO Favorite()VALUES()添加
DELETE Favorite  WHERE   删除
UPDATE Favorite SET    修改
select * from 表   查询

只查看的数重复
select * from users where utime  in
(
select max(utime) from users group by uid
having(count(1)>1)
)

去重复查看全部
select * from users where utime  in
(
select max(utime) from users group by uid
)

去重复查看全部
select a.* from
users a inner join
(select uid,max(utime)
as utime from users group by (uid) ) b on a.uid = b.uid and a.utime = b.utime

用泛型~可以防止变量出问题~安全性高
所谓泛型是指将类型参数化以达到代码复用提高软件开发工作效率的一种数据类型。
c# java是强类型语言
javascript是弱语言~它对类型不强求~如它申请变量是用var~然后不管什么类型！他都能识别
而C#和java~~它是强类型~~int型就是int型
虽然c#3.0有var
它不是相当于一种类型~它编译成机器语言的话~也会自动识别~如var i = 2;  c#会把i改成int型
所以它跟javascript不一样的