Aggregate column text where dates in table a are between dates in table b - sql

Question

Sample data
CREATE TEMP TABLE a AS
SELECT id, adate::date, name
FROM ( VALUES
(1,'1/1/1900','test'),
(1,'3/1/1900','testing'),
(1,'4/1/1900','testinganother'),
(1,'6/1/1900','superbtest'),
(2,'1/1/1900','thebesttest'),
(2,'3/1/1900','suchtest'),
(2,'4/1/1900','test2'),
(2,'6/1/1900','test3'),
(2,'7/1/1900','test4')
) AS t(id,adate,name);
CREATE TEMP TABLE b AS
SELECT id, bdate::date, score
FROM ( VALUES
(1,'12/31/1899', 7 ),
(1,'4/1/1900' , 45),
(2,'12/31/1899', 19),
(2,'5/1/1900' , 29),
(2,'8/1/1900' , 14)
) AS t(id,bdate,score);
What I want
What I need to do is aggregate column text from table a where the id matches table b and the date from table a is between the two closest dates from table b. Desired output:
id date score textagg
1 12/31/1899 7 test, testing
1 4/1/1900 45 testinganother, superbtest
2 12/31/1899 19 thebesttest, suchtest, test2
2 5/1/1900 29 test3, test4
2 8/1/1900 14
My thoughts are to do something like this:
create table date_join
select a.id, string_agg(a.text, ','), b.*
from tablea a
left join tableb b
on a.id = b.id
*having a.date between b.date and b.date*;
but I am really struggling with the last line, figuring out how to aggregate only where the date in table b is between the closest two dates in table b. Any guidance is much appreciated.

Answer 1

I can't promise it's the best way to do it, but this is a way to do it.
with b_values as (
select
id, date as from_date, score,
lead (date, 1, '3000-01-01')
over (partition by id order by date) - 1 as thru_date
from b
)
select
bv.id, bv.from_date, bv.score,
string_agg (a.text, ',')
from
b_values as bv
left join a on
a.id = bv.id and
a.date between bv.from_date and bv.thru_date
group by
bv.id, bv.from_date, bv.score
order by
bv.id, bv.from_date
I'm presupposing you will never have a date in your table greater than 12/31/2999, so if you're still running this query after that date, please accept my apologies.
Here is the output I got when I ran this:
id from_date score string_agg
1 0 7 test,testing
1 92 45 testinganother,superbtest
2 0 19 thebesttest,suchtest,test2
2 122 29 test3,test4
2 214 14
I might also note that between in a join is a performance killer. IF you have large data volumes, there might be better ideas on how to approach this, but that depends largely on what your actual data looks like.