hive里的group by和distinct

前言

今天才明确知道group by实际上还是有去重读作用的，其实细想一下，按照xx分类，肯定相同的就算是一类了，也就相当于去重来，详细的看一下。

group by

• 看一下实例1：

hive> select * from test;

OK

zhao	15	20170807

zhao	14	20170809

zhao	15	20170809

zhao	16	20170809

hive> select name from test;

OK

zhao

zhao

zhao

zhao

hive> select name from test group by name;

...

OK

zhao

Time taken: 40.273 seconds, Fetched: 1 row(s)

按照这个去分类，最后结果只有一个，达到了去重的效果；实际上，所谓去重，肯定是两个一样的才可以去重，下面试一下两列的效果：

hive> select name,age from test group by name,age;

...

OK

zhao	14

zhao	15

zhao	16

Time taken: 36.943 seconds, Fetched: 3 row(s)

hive> select name,age from test group by name;

FAILED: SemanticException [Error 10025]: Line 1:12 Expression not in GROUP BY key 'age'

只group by name就会出错，想一下只用name去做那么age不同就没法处理了，也合情合理。

distinct

这个也比较简单，就是去重：

hive> select distinct name from test;

...

OK

zhao

Time taken: 37.047 seconds, Fetched: 1 row(s)

hive> select distinct name,age from test;

OK

zhao	14

zhao	15

zhao	16

Time taken: 39.131 seconds, Fetched: 3 row(s)

hive> select distinct(name),age from test;

OK

zhao	14

zhao	15

zhao	16

Time taken: 37.739 seconds, Fetched: 3 row(s)

区别

• 如果数据较多，distinct效率会更低一些，一般推荐使用group by。
• 至于原因，推荐这篇文章