Lateral View语法

心已赠人 2022-08-08 12:49 3阅读 0赞

##     ##

## **Lateral View语法** ##

lateralView: LATERAL VIEW udtf(expression) tableAlias AS columnAlias (',' columnAlias)*
    fromClause: FROM baseTable (lateralView)*

## **描述** ##

lateral view用于和split, explode等UDTF一起使用，它能够将一行数据拆成多行数据，在此基础上可以对拆分后的数据进行聚合。lateral view首先为原始表的每行调用UDTF，UTDF会把一行拆分成一或者多行，lateral view再把结果组合，产生一个支持别名表的虚拟表。

## **例子** ##

假设我们有一张表pageAds，它有两列数据，第一列是pageid string，第二列是adid\_list，即用逗号分隔的广告ID集合：

<table> 
 <tbody> 
  <tr> 
   <td>string pageid</td> 
   <td>Array&lt;int&gt; adid_list</td> 
  </tr> 
  <tr> 
   <td>"front_page"</td> 
   <td>[1, 2, 3]</td> 
  </tr> 
  <tr> 
   <td>"contact_page"</td> 
   <td>[3, 4, 5]</td> 
  </tr> 
 </tbody> 
</table>

要统计所有广告ID在所有页面中出现的次数。

首先分拆广告ID：

SELECT pageid, adid 
        FROM pageAds LATERAL VIEW explode(adid_list) adTable AS adid;

执行结果如下：

<table> 
 <tbody> 
  <tr> 
   <td>string pageid</td> 
   <td>int adid</td> 
  </tr> 
  <tr> 
   <td>"front_page"</td> 
   <td>1</td> 
  </tr> 
  <tr> 
   <td>"front_page"</td> 
   <td>2</td> 
  </tr> 
  <tr> 
   <td>"front_page"</td> 
   <td>3</td> 
  </tr> 
  <tr> 
   <td>"contact_page"</td> 
   <td>3</td> 
  </tr> 
  <tr> 
   <td>"contact_page"</td> 
   <td>4</td> 
  </tr> 
  <tr> 
   <td>"contact_page"</td> 
   <td>5</td> 
  </tr> 
 </tbody> 
</table>

接下来就是一个聚合的统计：

SELECT adid, count(1) 
        FROM pageAds LATERAL VIEW explode(adid_list) adTable AS adid
    GROUP BY adid;

执行结果如下：

<table> 
 <tbody> 
  <tr> 
   <td>int adid</td> 
   <td>count(1)</td> 
  </tr> 
  <tr> 
   <td>1</td> 
   <td>1</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>1</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>2</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>1</td> 
  </tr> 
  <tr> 
   <td>5</td> 
   <td>1</td> 
  </tr> 
 </tbody> 
</table>

##     ##

## **多个lateral view语句** ##

一个FROM语句后可以跟多个lateral view语句，后面的lateral view语句能够引用它前面的所有表和列名。 以下面的表为例：

SELECT myCol1, col2 FROM baseTable
        LATERAL VIEW explode(col1) myTable1 AS myCol1;

<table> 
 <tbody> 
  <tr> 
   <td> <p>Array&lt;int&gt; col1</p> </td> 
   <td>Array&lt;string&gt; col2</td> 
  </tr> 
  <tr> 
   <td>[1, 2]</td> 
   <td>[a", "b", "c"]</td> 
  </tr> 
  <tr> 
   <td>[3, 4]</td> 
   <td>[d", "e", "f"]</td> 
  </tr> 
 </tbody> 
</table>

执行结果为：

<table> 
 <tbody> 
  <tr> 
   <td>int mycol1</td> 
   <td>Array&lt;string&gt; col2</td> 
  </tr> 
  <tr> 
   <td>1</td> 
   <td>[a", "b", "c"]</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>[a", "b", "c"]</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>[d", "e", "f"]</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>[d", "e", "f"]</td> 
  </tr> 
 </tbody> 
</table>

加上一个lateral view：

SELECT myCol1, myCol2 FROM baseTable
        LATERAL VIEW explode(col1) myTable1 AS myCol1
        LATERAL VIEW explode(col2) myTable2 AS myCol2;

它的执行结果为：

<table> 
 <tbody> 
  <tr> 
   <td>int myCol1</td> 
   <td>string myCol2</td> 
  </tr> 
  <tr> 
   <td>1</td> 
   <td>"a"</td> 
  </tr> 
  <tr> 
   <td>1</td> 
   <td>"b"</td> 
  </tr> 
  <tr> 
   <td>1</td> 
   <td>"c"</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>"a"</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>"b"</td> 
  </tr> 
  <tr> 
   <td>2</td> 
   <td>"c"</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>"d"</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>"e"</td> 
  </tr> 
  <tr> 
   <td>3</td> 
   <td>"f"</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>"d"</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>"e"</td> 
  </tr> 
  <tr> 
   <td>4</td> 
   <td>"f"</td> 
  </tr> 
 </tbody> 
</table>

注意上面语句中，两个lateral view按照出现的次序被执行。

转自 [http://www.cnblogs.com/ggjucheng/archive/2013/01/03/2842938.html][http_www.cnblogs.com_ggjucheng_archive_2013_01_03_2842938.html]

create table pageAds(pageid string,adid_list array<int>) row format delimited
    fields terminated by '\t' collection items terminated by ':';
    
    导进 的数据：
    front_page	1:2:3
    contact_page	3:4:5
    
    
    select * from pageAds;  不走mr
    front_page	[1,2,3]
    contact_page	[3,4,5]
    
    
    select pageid,xunilie from pageAds lateral view explode(adid_list) num as new_num;
    走mr

[http_www.cnblogs.com_ggjucheng_archive_2013_01_03_2842938.html]: https://cwiki.apache.org/confluence/display/Hive/LanguageManual+LateralView