Array Search in a CASE WHEN in SQL

Question

I currently have two tables worth of data. I am joining the two tables on multiple criteria based on certain findings.

CASE WHEN a.trans_dt = b.trans_dt and b.trans_type = 'MASSTS' THEN b.trans_details

My problem is sometimes there may be 2 even three of the trans_Type, I tried using MAX

    MAX(CASE WHEN a.trans_dt = b.trans_dt and b.trans_type = 'MASSTS' 
    THEN b.trans_details ELSE NULL END) AS trans_Dtls

but that didn't always work.

There is a field in the table b, that is a seq number and I want the trans type with the greater seq number as that was the last one that occurred that day.

CASE WHEN a.trans_dt = b.trans_dt and b.trans_type = 'MASSTS' 
THEN b.trans_Details(with greatest b.seq_num) 

I am not sure how to tell it to grab the largest one. I can't necessarily say last because of the way the data gets ingested the largest seq_num could be the first row or the last row or anywhere in between as they are not in any order when ingested into the database.

Answer 1

You can use window function.

select a.blah.., b.blah...
from a
left join 
(select b.* , row_number() over ( partition by  trans_type, trans_dt order by trans_type, trans_dt, seq_num desc) as rn from b ) as b
ON b.rn=1 and a.trans_dt = b.trans_dt and b.trans_type = 'MASSTS' --This rn will give you data for maximum row from table b. Pls note to join on correct columns between a and b

Explanation - Based on below two parameters, each window rows will be given rownumber like 1,2,3...
partition by trans_type, trans_dt - This will decide your window of data.
order by trans_type, trans_dt, seq_num desc - This will order the data inside above window. So, seq num with max seq will be numberd as 1.