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需求：

查询一批用户最后三次登陆时间，ip数据

理解需求是实现分组取前n个值，实现方式是先按照uid字段升序或倒序，时间字段倒序排序数据集合，然后遍历数据集合，用row_number函数遍历uid字段，相同则row_number值+1，取row_number<=3

row_number实现

import org.apache.hadoop.hive.ql.exec.UDF;

public class RowNumber extends UDF { 
   

    private static int MAX_VALUE = 50;
    private static String comparedColumn[] = new String[MAX_VALUE];
    private static int rowNum = 1;

    public int evaluate(Object... args) {
        String columnValue[] = new String[args.length];
        for (int i = 0; i < args.length; i++){
            columnValue[i] = args[i].toString();
        }

        if (rowNum == 1) {
            for (int i = 0; i < columnValue.length; i++)
                comparedColumn[i] = columnValue[i];
        }

        for (int i = 0; i < columnValue.length; i++) {
            if (!comparedColumn[i].equals(columnValue[i])) {
                for (int j = 0; j < columnValue.length; j++) {
                    comparedColumn[j] = columnValue[j];
                }
                rowNum = 1;
                return rowNum++;
            }
        }
        return rowNum++;
    }
}

使用

add jar /xx/xx/user_function/mr-function-1.0.0.jar;
create temporary function row_number as 'com.xxx.xxx.RowNumber';

SELECT c.user_id,c.time,c.ip FROM (SELECT b.* FROM uids_20150831 a LEFT JOIN (SELECT * FROM login WHERE date >=20150101) b ON a.user_id = b.user_id distribute BY user_id sort BY user_id ASC,time DESC ) c WHERE row_number(c.user_id)<=3

参考：
http://blog.csdn.net/liuj2511981/article/details/8741276
http://blog.csdn.net/yfkiss/article/details/7885262
http://blog.csdn.net/yfkiss/article/details/7885262