Spark PIVOT & UNPIVOT, 行转列和列转行

测试数据

name	course	score
Darren	Chinese	71
Darren	Math	81
Darren	English	91
Jonathan	Chinese	72
Jonathan	Math	82
Jonathan	English	92
Tom	Chinese	73

行转列

语法

SELECT
    xxx
FROM
    table_test
PIVOT(
    聚合函数(value_column) FOR pivot_column in ()
)

Example:

SELECT 
    * 
FROM row_table
PIVOT(
    MAX(score) FOR course in ('Chinese', 'Math', 'English')
)

结果：

name	Chinese	Math	English
Darren	71	81	91
Jonathan	72	82	92
Tom	73	null	null

列转行

spark并不支持UNPIVOT，而是用stack()来实现列转行

语法：

SELECT
    STACK
    (
        row_number, 
        'column1_value', column1_name,
         ..., 
        'columnn_value', columnn_name
    ) as (new_column1_name, new_column2_name)

Example:

SELECT
    name
  , STACK
        (
         3, 
         'Chinese', Chinese, 
         'Math', Math, 
         'English', English
    ) as (course, score)
FROM col_table

结果：

name	course	score
Darren	Chinese	71
Darren	Math	81
Darren	English	91
Jonathan	Chinese	72
Jonathan	Math	82
Jonathan	English	92
Tom	Chinese	73
Tom	Math	null
Tom	English	null

注意：此时发现结果表和最原始的表比较，Tom多了两行值为null，所以应该再过滤掉null值就得到了和原来一样的表

spark.sql(f"""
    SELECT
        name
      , STACK
            (
            3, 
            'Chinese', Chinese, 
            'Math', Math, 
            'English', English
        ) as (course, score)
    FROM col_table
    
""").where("score is not null")

就能得到和原来一样的结果了。

 

参考：

https://queirozf.com/entries/spark-dataframe-examples-pivot-and-unpivot-data

https://sparkbyexamples.com/spark/how-to-pivot-table-and-unpivot-a-spark-dataframe/