Question

I have a table with the following schema:

CREATE TABLE table (
  msg_id TEXT,
  thread_id TEXT,
  .
  .
  .
  date INTEGER 
)

I need to retrieve the most recent n msg_id per unique value of thread_id. Is there a way to do it using a single query or will I need to query the database to get the most recent distinct thread_ids, then query the database again PER unique thread_id? I recall reading somewhere that multiple database queries can get expensive.

Answer 1

You could use a correlated subquery. For example, for N = 5 :

select  *
from    YourTable yt1
where   5 <
        (
        select  count(*)
        from    YourTable yt2
        where   yt2.thread_id = yt1.thread_id
                and yt2.msg_id < yt1.thread_id
        )

This is not too fast, so you might be better of with multiple queries.