【学习笔记】hive 之行拆列explode

1、explode

explode(ARRAY) 列表中的每个元素生成一行
explode(MAP) map中每个key-value对，生成一行，key为一列，value为一列

限制：
1、No other expressions are allowed in SELECT

    SELECT pageid, explode(adid_list) AS myCol... is not supported

2、UDTF's can't be nested

    SELECT explode(explode(adid_list)) AS myCol... is not supported

3、GROUP BY / CLUSTER BY / DISTRIBUTE BY / SORT BY is not supported

    SELECT explode(adid_list) AS myCol ... GROUP BY myCol is not supported

2、lateral view

可使用lateral view解除以上限制，语法：

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

案例：

table名称为pageAds

SELECT pageid, adid

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

输出结果：

3、多个lateral view

from语句后面可以带多个lateral view语句

案例：

表名：baseTable

from后只有一个lateral view：

SELECT myCol1, col2 FROM baseTable

LATERAL VIEW explode(col1) myTable1 AS myCol1;

结果：

多个lateral view：

SELECT myCol1, myCol2 FROM baseTable

LATERAL VIEW explode(col1) myTable1 AS myCol1

LATERAL VIEW explode(col2) myTable2 AS myCol2;

结果：

4、Outer Lateral Views

如果array类型的字段为空，但依然需返回记录，可使用outer关键词。

比如：select * from src LATERAL VIEW explode(array()) C AS a limit 10;

这条语句中的array字段是个空列表，这条语句不管src表中是否有记录，结果都是空的。

而：select * from src LATERAL VIEW OUTER explode(array()) C AS a limit 10;

结果中的记录数为src表的记录数，只是a字段为NULL。

比如：

238 val_238 NULL
86 val_86 NULL
311 val_311 NULL
27 val_27 NULL
165 val_165 NULL
409 val_409 NULL
255 val_255 NULL
278 val_278 NULL
98 val_98 NULL

官方文档：

https://cwiki.apache.org/confluence/display/Hive/LanguageManual+LateralView

https://cwiki.apache.org/confluence/display/Hive/LanguageManual+UDF#LanguageManualUDF-explode