首页 > 代码库 > Lateral View 语法
Lateral View 语法
Lateral View 语法
1 lateralView: LATERAL VIEW udtf(expression) tableAlias AS columnAlias (‘,‘ columnAlias)*2 fromClause: FROM baseTable (lateralView)*
Lateral View 描述
Lateral View 用于UDTF(user-defined table generating functions)中将行转成列,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。
目前Lateral View 不支持有上而下的优化。如果使用Where子句,查询可能将不被编译。解决方法:在查询之前执行set hive.optimize.ppd=false
例子
表pageAds。它有两个列:pageid、adid_list:
Column name | Column type |
---|---|
pageid | STRING |
adid_list | Array<int> |
pageid | adid_list |
---|---|
front_page | [1, 2, 3] |
contact_page | [3, 4, 5] |
执行语句:
1 SELECT pageid, adid2 FROM pageAds LATERAL VIEW explode(adid_list) adTable AS adid;
将输出如下结果:
pageid (string) | adid (int) |
---|---|
"contact_page" | 3 |
"contact_page" | 4 |
"contact_page" | 5 |
"front_page" | 1 |
"front_page" | 2 |
"front_page" | 3 |
count/group可以被使用:
1 SELECT adid, count(1)2 FROM pageAds LATERAL VIEW explode(adid_list) adTable AS adid3 GROUP BY adid;
int adid | count(1) |
1 | 1 |
2 | 1 |
3 | 2 |
4 | 1 |
5 | 1 |
多个Lateral View
例如下面的语句:
1 SELECT * FROM exampleTable2 LATERAL VIEW explode(col1) myTable1 AS myCol13 LATERAL VIEW explode(myCol1) myTable2 AS myCol2;
假设有base表如下:
Array<int> col1 | Array<string> col2 |
[1, 2] | [a", "b", "c"] |
[3, 4] | [d", "e", "f"] |
语句:
1 SELECT myCol1, col2 FROM baseTable2 LATERAL VIEW explode(col1) myTable1 AS myCol1;
将得到如下结果:
int mycol1 | Array<string> col2 |
1 | [a", "b", "c"] |
2 | [a", "b", "c"] |
3 | [d", "e", "f"] |
4 | [d", "e", "f"] |
多个 LATERAL VIEW 的语句:
1 SELECT myCol1, myCol2 FROM baseTable2 LATERAL VIEW explode(col1) myTable1 AS myCol13 LATERAL VIEW explode(col2) myTable2 AS myCol2;
结果如下:
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" |