SQL查询重复数据和清除重复数据
2014-08-05 10:39
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选择重复,消除重复和选择出序列
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有例表:emp
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emp_no name age
001 Tom 17
002 Sun 14
003 Tom 15
004 Tom 16
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要求:
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列出所有名字重复的人的记录
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(1)最直观的思路:要知道所有名字有重复人资料,首先必须知道哪个名字重复了:
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select name from emp group by name having count(*)>1
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所有名字重复人的记录是:
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select * from emp
where name in (select name from emp group by name having count(*)>1)
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(2)稍微再聪明一点,就会想到,如果对每个名字都和原表进行比较,大于2个人名字与这条记录相同的就是合格的 ,就有
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select * from emp where (select count(*) from emp e where e.name=emp.name) >1
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--注意一下这个>1,想下如果是 =1,如果是 =2 如果是>2 如果 e 是另外一张表 而且是=0那结果 就更好玩了:)
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这个过程是 在判断工号为001的 人 的时候先取得 001的 名字(emp.name) 然后和原表的名字进行比较 e.name
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注意e是emp的一个别名。
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再稍微想得多一点,就会想到,如果有另外一个名字相同的人工号不与她他相同那么这条记录符合要求:
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select * from emp
where exists
(select * from emp e where e.name=emp.name and e.emp_no<>emp.emp_no)
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此思路的join写法:
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select emp.* from emp,emp e
where emp.name=e.name and emp.emp_no<>e.emp_no
/* 这个语句较规范的 join 写法是
select emp.* from emp inner join emp e on emp.name=e.name and emp.emp_no<>e.emp_no
但个人比较倾向于前一种写法,关键是更清晰 */
b、有例表:emp
name age
Tom 16
Sun 14
Tom 16
Tom 16
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----------------------------------------------------清除重复----------------------------------------------------
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过滤掉所有多余的重复记录
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(1)我们知道distinct、group by 可以过滤重复,于是就有最直观的
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select distinct * from emp 或 select name,age from emp group by name,age
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获得需要的数据,如果可以使用临时表就有解法:
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select distinct * into #tmp from emp
delete from emp
insert into emp select * from #tmp
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(2)但是如果不可以使用临时表,那该怎么办?
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我们观察到我们没办法区分数据(物理位置不一样,对 SQL Server来说没有任何区别),思路自然是想办法把数据区分出来了,既然现在的所有的列都没办法区分数据,唯一的办法就是再加个列让它区分出来,加什么列好?最佳选择是identity列:
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alter table emp add chk int identity(1,1)
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表示例:
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name age chk
Tom 16 1
Sun 14 2
Tom 16 3
Tom 16 4
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重复记录可以表示为:
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select * from emp where (select count(*) from emp e where e.name=emp.name)>1
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要删除的是:
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delete from emp
where (select count(*) from emp e where e.name=emp.name and e.chk>=emp.chk)>1
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再把添加的列删掉,出现结果。
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alter table emp drop column chk
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(3)另一个思路:
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视图
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select min(chk) from emp group by name having count(*) >1
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获得有重复的记录chk最小的值,于是可以
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delete from emp where chk not in (select min(chk) from emp group by name)
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写成join的形式也可以:
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(1)有例表:emp
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![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
emp_no name age
001 Tom 17
002 Sun 14
003 Tom 15
004 Tom 16
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![](http://images.csdn.net/syntaxhighlighting/OutliningIndicators/None.gif)
◆要求生成序列号
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(1)最简单的方法,根据b问题的解法:
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alter table emp add chk int identity(1,1) 或
select *,identity(int,1,1) chk into #tmp from emp
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◆如果需要控制顺序怎么办?
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select top 100000 *,identity(int,1,1) chk into #tmp from emp order by age
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(2) 假如不可以更改表结构,怎么办?
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如果不可以唯一区分每条记录是没有办法的,在可以唯一区分每条记录的时候,可以使用a 中的count的思路解决这个问题
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select emp.*,(select count(*) from emp e where e.emp_no<=emp.emp_no)
from emp
order by (select count(*) from emp e where e.emp_no<=emp.emp_no)
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