Exchange rate table and conversion on the fly in query I'm working on a small project and I'm have arrived to the point where I let a user chose in which currency to display items.

I'm working with Java and my first thought was that I would use some kind of API to pull the rates and post process (loop) after I get the the items from the DB. But I'd rather not have to unnecessarily loop if I can directly get the right price form the query.

So I think I'm going to store all the exchanges rate in the DB and have my query do the conversation job here is my first draft:

 CREATE TABLE currency (
      code VARCHAR(3) NOT NULL,
      symbol VARCHAR(3) NOT NULL,
      display_name VARCHAR(4) NOT NULL,
      PRIMARY KEY (code),
      UNIQUE KEY uq_ccode (code)
    );

CREATE TABLE exchange_rate (
  currency_code_from VARCHAR(3) NOT NULL,
  currency_code_to VARCHAR(3) NOT NULL,
  rate numeric(15,4) NOT NULL,
  created timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  updated timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (currency_code_from, currency_code_to),
  FOREIGN KEY (currency_code_from) REFERENCES currency(code) ON DELETE CASCADE,
  FOREIGN KEY (currency_code_to) REFERENCES currency(code) ON DELETE CASCADE
);


I would then write a query like:

select price as originalPrice, (price * rate) as TargetPrice FROM .....
JOIN exchange_rate er ON er.currency_code_from=(the currency of the item) AND er.currency_code_to=(the user currency)......


Note that on the above tables I would have to have every single combination

GBPEUR
EURGBP
GBPGBP (well i guess i can probably avoid this one if a slighty amend the query to detect this scenario)


Plus.. I would have to have some kind of cron job to daily update this table.

Now I'm far to be a banker nor a SQL genius so I would like some input from people that have already written such tables/query and that have expertise in this kind of stuff

Am I fare off with my design? Would I be better off post processing the results and do the conversation in java  with rates coming from an API?
How are your underlying item prices stored?

If, for example, your items are all priced/stored in EUR, then there is no need for an NxN (duplicative) currency matrix.   

In this case, exchange_rate (against EUR or your base currency) can simply be a field in the currency table (vs an independent table).

Then all of your conversions for display (which are all then simply EUR->XXX) can be calculated with a single JOIN.