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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。
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:
string pageid | Array<int> adid_list |
"front_page" | [1, 2, 3] |
"contact_page" | [3, 4, 5] |
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:
string pageid | int adid |
"front_page" | 1 |
"front_page" | 2 |
"front_page" | 3 |
"contact_page" | 3 |
"contact_page" | 4 |
"contact_page" | 5 |
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:
int adid | count(1) |
1 | 1 |
2 | 1 |
3 | 2 |
4 | 1 |
5 | 1 |
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:
Array<int> col1 | Array<string> col2 |
[1, 2] | [a", "b", "c"] |
[3, 4] | [d", "e", "f"] |
SELECT myCol1, col2 FROM baseTable
LATERAL VIEW explode(col1) myTable1 AS myCol1;
执行结果为:
int mycol1 | Array<string> col2 |
1 | [a", "b", "c"] |
2 | [a", "b", "c"] |
3 | [d", "e", "f"] |
4 | [d", "e", "f"] |
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:
int myCol1 | string myCol2 |
1 | "a" |
1 | "b" |
1 | "c" |
2 | "a" |
2 | "b" |
2 | "c" |
3 | "d" |
3 | "e" |
3 | "f" |
4 | "d" |
4 | "e" |
4 | "f" |
注意上面语句中,两个lateral view按照出现的次序被执行。
转自 https://cwiki.apache.org/confluence/display/Hive/LanguageManual+LateralView#
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