方法一:
在使用MySQL时,有时需要查询出某个字段不重复的记录,虽然mysql提供 有distinct这个关键字来过滤掉多余的重复记录只保留一条,但往往只用它来返回不重复记录的条数,而不是用它来返回不重记录的所有值。其原因是 distinct只能返回它的目标字段,而无法返回其它字段
下面先来看看例子:
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table
id name
1 a
2 b
3 c
4 c
5 b
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库结构大概这样,这只是一个简单的例子,实际情况会复杂得多。
比如我想用一条语句查询得到name不重复的所有数据,那就必须使用distinct去掉多余的重复记录。
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select distinct name from table
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得到的结果是:
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name
a
b
c
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好像达到效果了,可是,我想要得到的是id值呢?改一下查询语句吧:
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select distinct name , id from table
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结果会是:
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id name
1 a
2 b
3 c
4 c
5 b
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distinct怎么没起作用?作用是起了的,不过他同时作用了两个字段,也就是必须得id与name都相同的才会被除。。。。。。。
我们再改改查询语句:
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select id, distinct name from table
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很遗憾,除了错误信息你什么也得不到,distinct必须放在开头。难到不能把distinct放到where条件里?能报错。。。。。。。
最终好用的语句如下:
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select *, count ( distinct name ) from table group by name
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结果:
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id name count ( distinct name )
1 a 1
2 b 1
3 c 1
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最后一项是多余的,不用管就行了,目的达到。。。。。
哦,对,再顺便说一句,group by 必须放在 order by 和 limit之前,不然会报错。。。。。。。。!OK了
总结语句:select *, count(distinct name) from (select * from table……等嵌套语句) group by name
方法二:
利用group by
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SELECT * FROM (
select * from customer where user =(
SELECT source_user from customer WHERE user = 'admin' ) UNION ALL select * from customer where user =(
select source_user from customer where user =(
SELECT source_user from customer WHERE user = 'admin' )) union ALL select * from customer where user =(
select source_user from customer where user =(
select source_user from customer where user =(
SELECT source_user from customer WHERE user = 'admin' ))) UNION ALL select * from customer where source_user=(/*我的上线的上线的 user */
select user from customer where user =(
select source_user from customer where user =(
SELECT source_user from customer WHERE user = 'admin' ))) union all select * from customer where source_user=(/*我的上线的上线的上线 user */
select user from customer where user =(
select source_user from customer where user =(
select source_user from customer where user =(
SELECT source_user from customer WHERE user = 'admin' ))))) as alias group by user ;
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注意加别名,不然报错,注意在where语句外面包装一下,再用group by去重才会生效。
感谢阅读,希望能帮助到大家,谢谢大家对本站的支持!
原文链接:http://blog.csdn.net/djun100/article/details/10452165