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<>sql字段去重
在从数据库中取数据或者进行数据清洗的过程中，经常会遇到去重复问题，常用的去重方法包括三种方式。接下来主要介绍常用的三种方式以及最优方法。
* distinct
* group by
* row_number() over()
<>需求分析
对下表中uid字段去重，并且把去重后所有字段输出。
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<>distinct方法
* ditinct方法适合于单字段去重，但是单字段去重还要保留其他字段数据，就无法完成了；
* distinct对NULL是不进行过滤的，即返回的结果中是包含NULL值的；
* distinct应用到多个字段的时候，distinct必须放在开头，其应用的范围是其后面的所有字段 ---
<>group by 方法
* 对group by 后面所有字段去重，并不能只对一列去重；
* sql语句写成只对一列去重，保留其他字段，在hive上会报错
报错结果如下：
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<>row_number() over()（该方法是hive sql去重的最佳方法）
* ROW_Number() over (partition by id order by time DESC)
给分组后的每个id加一列按某一字段倒叙排序的rank值，取rank=1 SELECT aa.uid,aa.org_name,aa.addr_no,
row_number() over (partition by aa.uid order BY aa.addr_no) as rn FROM stg.
t_018_doc_address_spec_organizationAS aa ) AS t WHERE t.rn =1
上述方法去重结果如下：
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