• explode函式：hive內建的表生成函式，主要用於將一行輸入拆分成多行輸出。

官網解釋：explode() takes in an array (or a map) as an input and outputs the elements of the array (map) as separate rows. UDTFs can be used in the SELECT expression list and as a part of LATERAL VIEW. ①建立array資料型別表

hive> create table student_array(
    > name string,
    > course_score array<string>
    > )
    > row format delimited fields terminated by '\t'
    > collection items terminated by ','
    > stored as textfile;
 

②準備資料student_array.txt

[email protected]:/home/hadoop/dataset# cat student_array.txt
zs	語文:86,數學:87.5,英語:90
ls	語文:76,數學:93,英語:88
ww	語文:88,數學:90,英語:95

③將資料匯入到hive表

hive> load data local inpath '/home/hadoop/dataset/student_array.txt' overwrite into table student_array;

④檢視hive表資料

hive> select * from student_array;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
zs	["語文:86","數學:87.5","英語:90"]
ls	["語文:76","數學:93","英語:88"]
ww	["語文:88","數學:90","英語:95"]
 

對於array資料型別，explode函式將陣列中的元素拆分，按行輸出每個元素

hive> select explode(course_score) from student_array;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
語文:86
數學:87.5
英語:90
語文:76
數學:93
英語:88
語文:88
數學:90
英語:95

posexplode函式類似於explode函式，只是在返回陣列中的元素同時也返回元素在陣列中的索引位置

hive> select posexplode(course_score)   from  student_array;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
0	語文:86
1	數學:87.5
2	英語:90
0	語文:76
1	數學:93
2	英語:88
0	語文:88
1	數學:90
2	英語:95
 

對於map資料型別，explode函式會將一個map資料型別拆分成兩列輸出，鍵一列，值一列。 首先建立帶map的表

hive> create table student_map(
    > name string,
    > course_score map<string ,float>
    > )
    > row format delimited fields terminated by '\t'
    > collection items terminated by ','
    > map keys terminated by ':'
    > stored as textfile;
hive> select * from student_map;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
zs	{"語文":86.0,"數學":87.5,"英語":90.0}
ls	{"語文":76.0,"數學":93.0,"英語":88.0}
ww	{"語文":88.0,"數學":90.0,"英語":95.0}

explode函式將map資料型別的資料鍵值對拆分長兩列

hive> select explode(course_score) from student_map;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
語文	86.0
數學	87.5
英語	90.0
語文	76.0
數學	93.0
英語	88.0
語文	88.0
數學	90.0
英語	95.0

posexplode函式的引數只能是陣列，不能是map資料型別

hive> select posexplode(course_score) from student_map;
FAILED: UDFArgumentException posexplode() takes an array as a parameter

對於結構體陣列，可以使用inline函式進行拆分

hive> select * from student_struct_array;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
zs	[{"course":"語文","score":86.0},{"course":"數學","score":87.5},{"course":"英語","score":90.0}]
ls	[{"course":"語文","score":76.0},{"course":"數學","score":93.0}]
ww	[{"course":"語文","score":88.0},{"course":"數學","score":90.0},{"course":"英語","score":95.0}]
Time taken: 0.19 seconds, Fetched: 3 row(s)
hive> select inline(course_score) from student_struct_array;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
語文	86.0
數學	87.5
英語	90.0
語文	76.0
數學	93.0
語文	88.0
數學	90.0
英語	95.0

在select語句中使用UDTF語法的限制： ①在select語句中只支援單個UDTF表示式 ②不支援UDTF巢狀 ③不支援GROUP BY / CLUSTER BY / DISTRIBUTE BY / SORT BY語句

hive> select explode(name) , explode(course_score) from student_array;
FAILED: SemanticException 1:23 Only a single expression in the SELECT clause is supported with UDTF's. Error encountered near token 'course_score'
hive> select explode(name) , explode(course_score) from student_array;
FAILED: SemanticException 1:23 Only a single expression in the SELECT clause is supported with UDTF's. Error encountered near token 'course_score'
hive> select name ,explode(course_score) from student_array;
FAILED: SemanticException [Error 10081]: UDTF's are not supported outside the SELECT clause, nor nested in expressions
hive> select explode(course_score) from student_array sort by name;
FAILED: SemanticException [Error 10078]: SORT BY is not supported with a UDTF in the SELECT clause

explode函式和lateral view結合使用，就不存在上上語法限制

hive> select name ,course ,score from student_struct_array  lateral view inline(course_score)  course_scores as course , score ;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
zs	語文	86.0
zs	數學	87.5
zs	英語	90.0
ls	語文	76.0
ls	數學	93.0
ww	語文	88.0
ww	數學	90.0
ww	英語	95.0

lateral view outer 使用方法

hive> select * from student_array lateral view outer explode(array())  c as a;
-chgrp: '*' does not match expected pattern for group
Usage: hadoop fs [generic options] -chgrp [-R] GROUP PATH...
OK
zs	["語文:86","數學:87.5","英語:90"]	NULL
ls	["語文:76","數學:93","英語:88"]	NULL
ww	["語文:88","數學:90","英語:95"]	NULL
Time taken: 0.166 seconds, Fetched: 3 row(s)