SQL Query to combine multiple rows into one based on ID while keeping other value in the same row?

Question

first of all, I have been searching for this for some time.

I have a table that looks kinda like this:

ID        Expenditure  MonthYear
1A        1,000        122019
1A        1,500        012020
1B        1,900        122019
1C        2,400        122019
1B        2,400        012020
1C        900          012020
1A        800          022020

Since the rows can reach to thousands, and some IDs are repeated tens of times, I want to combine those with distinct ID into a single row and add columns which retain all the information in it. I want to make the table to looks something like this:

ID        Expenditure_1  MonthYear_1 Expenditure_2 MonthYear_2  Expenditure_3   MonthYear_3
1A        1,000          122019      1,500         012020       800             022020
1B        1,900          122019      2,400         012020       Null            Null
1C        2,400          122019      900           012020       Null            Null

What is the best way in approaching this problem using SQL on Impala? Thank you.

Answer 1

You can use conditional aggregation and row_number():

select id,
       max(case when seqnum = 1 then expenditure end) as expenditure_1,
       max(case when seqnum = 1 then monthyear end) as monthyear_1,
       max(case when seqnum = 2 then expenditure end) as expenditure_2,
       max(case when seqnum = 2 then monthyear end) as monthyear_2,
       max(case when seqnum = 3 then expenditure end) as expenditure_3,
       max(case when seqnum = 3 then monthyear end) as monthyear_3
from (select t.*,
             row_number() over (partition by id order by right(monthyear, 4), left(monthyear, 2)) as seqnum
      from t
     ) t
group by id;