hive中自定义函数(UDAF)实现多行字符串拼接为一行

函数如何使用：

hive> desc concat_test;

OK

a int

b string

hive> select * from concat_test;

OK

1 good

2 other

1 nice

1 hello

hive> select a,concat(b,',') from concat_test group by a;

OK

1 good,nice,hello

2 other

函数实现：

package com.hadoopbook.hive;

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

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

public class concat extends UDAF {

public static class ConcatUDAFEvaluator implements UDAFEvaluator{

public static class PartialResult{

String result;

String delimiter;

}

private PartialResult partial;

public void init() {

partial = null;

}

public boolean iterate(String value,String deli){

if (value == null){

return true;

}

if (partial == null){

partial = new PartialResult();

partial.result = new String("");

if( deli == null || deli.equals("") )

{

partial.delimiter = new String(",");

}

else

{

partial.delimiter = new String(deli);

}

}

if ( partial.result.length() > 0 )

{

partial.result = partial.result.concat(partial.delimiter);

}

partial.result = partial.result.concat(value);

return true;

}

public PartialResult terminatePartial(){

return partial;

}

public boolean merge(PartialResult other){

if (other == null){

return true;

}

if (partial == null){

partial = new PartialResult();

partial.result = new String(other.result);

partial.delimiter = new String(other.delimiter);

}

else

{

if ( partial.result.length() > 0 )

{

partial.result = partial.result.concat(partial.delimiter);

}

partial.result = partial.result.concat(other.result);

}

return true;

}

public String terminate(){

return new String(partial.result);

}

}

}