Order By语法
对全局数据的排序，只有一个reduce
colOrder: ( ASC | DESC )
colNullOrder: (NULLS FIRST | NULLS LAST) -- (Note: Available in Hive 2.1.0 and later)
orderBy: ORDER BY colName colOrder? colNullOrder? (',' colName colOrder? colNullOrder?)*
query: SELECT expression (',' expression)* FROM src orderBy
order by 样例
select * from emp;
Sort By
对每一个Reduce内部进行排序，对全局结果集来说不是排序的
设置 reduce 执行的个数
set mapreduce.job.reduces=<number>
sort by样例
hive>set mapreduce.job.reduces=3;
hive>insert overwrite local directory '/opt/datas/hive_exp_emp0308' ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' COLLECTION ITEMS TERMINATED BY '\n'
select * from emp sort by empno asc
Distribute By
也就是分区partition，类似MapReduce中分区partition，对数据进行分区后，结合sort by 进行排序使用。
insert overwrite local directory '/opt/datas/hive_exp_distribute_emp0308' ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' COLLECTION ITEMS TERMINATED BY '\n'
select * from emp distribute by deptno sort by empno asc
第一个分区数据000000_0
第二个分区000001_0
第三个分区000002_0
Cluster By
当sort by 和 distribute by的字段相同时，就可以使用Cluster By替换。
insert overwrite local directory '/opt/datas/hive_exp_cluster_emp0308' ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' COLLECTION ITEMS TERMINATED BY '\n'
select * from emp cluster by empno
总结
Hive中select新特性
Order By 全局排序，一个Reduce
Sort By 每个reduce内部进行排序，全局不是排序 Distribute By 类似MR中partition，进行分区，结合sort by使用 Cluster By 当distribute和sort字段相同时，使用方式
