Order By:全局排序,一个reduce
SELECT * FROM student s LEFT JOIN score sco ON s.s_id = sco.s_id ORDER BY sco.s_score DESC;
SELECT * FROM student s LEFT JOIN score sco ON s.s_id = sco.s_id ORDER BY sco.s_score asc;
按照分数的平均值排序
select s_id ,avg(s_score) avg from score group by s_id order by avg;
按照学生id和平均成绩进行排序
select s_id ,avg(s_score) avg from score group by s_id order by s_id,avg;
Sort By:每个MapReduce内部进行排序,对全局结果集来说不是排序。
set mapreduce.job.reduces=3;
set mapreduce.job.reduces;
select * from score sort by s_score;
insert overwrite local directory '/export/servers/hivedatas/sort' select * from score sort by s_score
Distribute By:类似MR中partition,进行分区,结合sort by使用。
注意,Hive要求DISTRIBUTE BY语句要写在SORT BY语句之前。
对于distribute by进行测试,一定要分配多reduce进行处理,否则无法看到distribute by的效果。
案例实操:先按照学生id进行分区,再按照学生成绩进行排序。
set mapreduce.job.reduces=7;
insert overwrite local directory '/export/servers/hivedatas/sort' select * from score distribute by s
当distribute by和sort by字段相同时,可以使用cluster by方式。
cluster by除了具有distribute by的功能外还兼具sort by的功能。但是排序只能是倒序排序,不能指定排序规则为ASC 或者DESC。 以下两种写法等价
select * from score cluster by s_id;
select * from score distribute by s_id sort by s_id;