Currencies, exchange rates, how to make proper relations?

Question

I have multiple currencies (they are not real) and exchange rates between them. Eg currency_a , currency_b , 1.5 If i need to exchange A -> B i simply multiply value by 1.5 , or if i need to exchange B -> A i divide value by 1.5 . How can i properly express this in database?

Answer 1

You would simply have a conversion table with two columns as a conversion factor:

from_currency  to_currency    rate     
      A            B          1.5
      B            A          0.66667

In this method, the rates are multiplicative and both pairs are stored. This should make it easy to use.

For performance, you want a primary key on (from_currency, to_currency) .

Answer 2

That approach IS a M:M it's just that both sides reference the same parent table. The PK remains the same, but each should also be an FK. A sample layout maybe something like:

create table currency ( 
             id_currency serial 
           , code        text
           , name        text
           , constraint  currency_pk
                         primary key (id_currency)
           , constraint  currency_bk 
                         unique (code)
           ) ; 
create table currency_conversion(
             from_currency integer 
           , to_currency   integer
           , rate          number(9,5)
           , constraint    currency_conversion_pk
                           primary key (from_currency,to_currency)
           , constraint    from_conversion_2_currency_fk
                           foreign key (from_currency)
                           references currency(id_currency)
           , constraint    to_conversion_2_currency_fk
                           foreign key (to_currency)
                           references currency(id_currency)
          );