Date selection of appropriate row

Question

We have a table that stores the date and currency rate applicable for that day. A new row is inserted each day. However, there may be times when we don't get a new row inserted for a day or two.
For example:
EFFECTIVE_DATE     USD    GBP
15/03/2010           1    0.56
16/03/2010          1    0.58
19/03/2010           1    0.55
20/03/2010           1    0.57I am trying to write a function that will say what the GBP rate was if a user supplies the date they're interested in. So if they run the code supplying '17/3/2010', I need to return 0.58, because that was the rate last set before that date, there being no actual row that explicitly applies to that date.
I obviously can't have code that says 'where effective_date = user_supplied_date', because there's no row that equals what the user supplies, necessarily. But I am stumped coming up with how I say the effective date must be greater or equal to the user_supplied_date and less than the next effective date in the table after the first one that meets that criteria.
Any clues, please?! Something tells me it's probably trivial, but I'm having a logic block over this one!

Answer 1

Thank you for both of these: I will certainly check them out.
I did have a logic moment in the shower this morning, however: surely, it's OK to simply select the maximum date which is less than or equal to the supplied date? If so, then this query would do the job, I think:
select max(effective_date) from
  (select effective_date from currency_rates where effective_date<='date_as_supplied_by_user');...and then, knowing which date should be considered really effective, it's trivial to select the correct GBP amount with a further round of nesting:
select gbp from currency_rates where effective_date= 
(select max(effective_date) from
  (select effective_date from currency_rates where effective_date<=to_date('18/03/2010','DD/MM/YYYY')));  If this is a serious logic fail on my part, I'd not be surprised, though. Your dense rank example is really nice, too.