Hadoop Apache Pig AVG函数

我正在尝试做一个简单的Pig查询，我需要找到id为178的电影的平均评分。我已经尝试了下面的一些版本，过滤器正在工作，但没有AVG功能。有人能提供建议吗？谢谢

a = load '/user/pig/u.data' AS (userid:int, movieid:int, rating:double, timestamp:chararray);  
b = FOREACH a GENERATE AVG(rating) as rate, movieid;
c = group b by rate;
d= filter a by movieid==178;
dump d;

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你应该按电影ID分组

b = FILTER a BY (movieid == 178);
c = GROUP b BY movied;
d = FOREACH c GENERATE group AS movieid,AVG(a.rating) as rate;

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首先筛选，然后分组，然后平均。感谢您的帮助！