问题
Assume i have a table like this,
table: qa_list
id | question_id | question | answer |
---------+--------------+------------+-------------
1 | 100 | question1 | answer |
2 | 101 | question2 | answer |
3 | 102 | question3 | answer |
4 | ...
... | ...
and a query that gives below result (since I couldn't find a direct way to transpose the table),
table: qa_map
id | qa_map
--------+---------
1 | {question1=answer,question2=answer,question3=answer, ....}
Where qa_map is the result of a map_agg of arbitrary number of questions and answers.
Is there a way to UNNEST qa_map to an arbitrary number of columns as shown below?
id | Question_1 | Answer_1 | Question_2 | Answer_2 | Question_3 | ....
---------+-------------+-----------+-------------+-----------+-------------+
1 | question | answer | question | answer | question | ....
AWS Athena/Presto-0.172
回答1:
No, there is no way to write a query that results in different number of columns depending on the data. The columns must be known before query execution starts. The map you have is as close as you are going to get.
If you include your motivation for wanting to do this there may be other ways we can help you achieve your end goal.
来源：https://stackoverflow.com/questions/62876977/athena-presto-unnest-map-to-columns