Should I use a subquery?

Question

I have two tables, one that stores the current price, and one that stores the historical price of items. I want to create a query that pulls the current price, and the difference between the current price and the most recent historical price.

In the historical table, I have the start and end times of the price, so I can just select the most recent price, but how do I pull it all together in one query? Or do I have to do a subquery?

select p.current_price,
h.historical_price
h.historical_time  

from price p

inner join price_history h
on p.id = h.id
where max(h.historical_time)

This obviously doesn't work, but that is what I'm trying to accomplish.

This gives me the current and historical price. But I want to make sure I have the most RECENT price. How would I do this?

Answer 1

I would do it like this. Note, you may get duplicate records if there are two price entries with the same date for the same id in price_history :

select p.current_price, h.historical_price,
    p.current_price - h.historical_price as PriceDeff, h.historical_time 
from price p
inner join (
    select id, max(historical_time) as MaxHistoricalTime
    from price_history
    group by id
) hm on p.id = hm.id
inner join price_history h on hm.id = h.id 
    and hm.MaxHistoricalTime = h.historical_time

Answer 2

I don't believe there's a way of doing this without a subquery that isn't worse. On the other hand, if your table is indexed correctly, subqueries returning results of aggregate functions are generally pretty fast.

Answer 3

select
      p.current_price,
      h3.historical_price,
      h3.historical_time
  from 
      price p,
      ( select h1.id, max( h1.historical_time ) as MaxHT
            from price_history h1
            group by 1 ) h2,
      price_history h3
  where 
          p.id = h2.id
      and p.id = h3.id
      and h2.MaxHT = h3.historical_time