主要参考：
https://book.itxueyuan.com/3b7D/XDBe
基础数据：
==============================================
[root@cdh1 data]# cat demodata
xiaoxiao,12,12.1f
aaa,13,1.1f
kjkj,12,12.1f
ddf,19,12.8f
youyou,89,12.3f
==============================================
grunt> A = load '/root/xytest/pig/data/demodata' using PigStorage(',') as (name:chararray,age:int,gpa:float);
grunt> B = order A by age DESC;
grunt> dump B;
输出结果：
(youyou,89,12.3)
(ddf,19,12.8)
(aaa,13,1.1)
(kjkj,12,12.1)
(xiaoxiao,12,12.1)
能够看到结果已经根据第二列倒叙排列了。
==============================================
主要参考：
https://book.itxueyuan.com/3b7D/BDOK
limit 主要用于限定输出数量
==============================================
[root@cdh1 data]# cat demodata
xiaoxiao,12,12.1f
aaa,13,1.1f
kjkj,12,12.1f
ddf,19,12.8f
youyou,89,12.3f
==============================================
grunt> A = load '/root/xytest/pig/data/demodata' using PigStorage(',') as (name:chararray,age:int,gpa:float);
grunt> B = limit A 3;
grunt> dump B;
运行结果：
(aaa,13,1.1)
(kjkj,12,12.1)
(xiaoxiao,12,12.1)
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