What is the best way to do this query (SQL)

Question

i need to do a query but i can't find a good way to do it efficiently.

Everytime a client listen a song, one line is registered with his key and the track_id, date....

I want to check for every clients if the track he listened the last month are new (never listened before this month so)

One line look like this:

key | track_id | date
asd | 12312    | 12/02/2020
fds | 12323    | 12/05/2020

etc

I think i can do something with window functions but i can't seem to find a good way to do it.

Then i also need to get the top 3 most listened song from this list, which i can just do with a window function i guess.

If someone can help me? Thank you very much.

Answer 1

Use aggregation and window functions:

select key, count(*) as num_rows,
       count(*) as num_tracks,
       sum( case when first_yyyymm = yyyymm then 1 else 0 end) as num_new_tracks,
       sum( case when first_yyyymm < yyyymm then 1 else 0 end) as num_prev_tracks
from (select t.key, track_id, date_trunc('month', date) as yyyymm,
             min(date_trunc('month', date) ) over (partition by key, track_id) as first_yyyymm
      from t
      group by key, track_id, yyyymm
     ) t
where yyyymm >= date_trunc('month', current_date)
group by key

Answer 2

one piece of advice. if you want help from people, always include the DDL for your sample data set. Its not fair to expect people to do it for you. (In this case I did)

create table sandbox.songs 
(key varchar,
track_id number,
dt date);

insert into  sandbox.songs 
values
('asd', 12312, '12/02/2020'),
('fds', 12323, '12/05/2020'),
('asd', 11000, '04/05/2020'),
('fds', 92823, '04/07/2020'),
('asd', 11000, '3/05/2020'),
('fds', 12323, '12/05/2020'),
('asd', 92834, '3/05/2020');

QUESTION 1

select 
    key, 
    count(0), 
    sum(case when dt < current_date - 30 then 1 else 0 end) as older_than_30,
    sum(case when dt >= current_date - 30 then 1 else 0 end) as within_30    
from sandbox.songs group by key;

[enter image description here][1]

QUESTION 2

track_id, 
count(0) 
from sandbox.songs 
group by track_id 
order by count(0) desc 
limit 3;

[enter image description here][2]


  [1]: https://i.stack.imgur.com/CeDqw.png
  [2]: https://i.stack.imgur.com/sdKV0.png