Primary and foreign keys database design - sql

Question

I am creating SQL Server tables for a currency exchange system.
I have one table which is the following:
CREATE TABLE [CurrencyKeys]
(
[Key] [nchar](3) NOT NULL,
[Currency] [nvarchar](30) NOT NULL
CONSTRAINT [PK_Key] PRIMARY KEY ([Key])
);
How would I create the table for the exchange rates itself which references the keys from the CurrencyKeys table?
Currently I have the following:
CREATE TABLE [ExchangeRates]
(
[DateTime] [datetime]NOT NULL,
[FromCurrCode] [nchar](3) NOT NULL,
[ToCurrCode] [nchar] (3) NOT NULL,
[Rate] [money] NOT NULL
)
Would I need to create (FromCurrCode, ToCurrCode) as primary key as well?

Answer 1

If I understand you correctly, you need to define the foreign keys on the exchangeRates table.
You would define a foreign key for EACH column, like so:
alter table [ExchangeRates] add constraint fk_ExchangeRates_01 foreign key (fromCurrCode) references CurrencyKeys([Key]);
alter table [ExchangeRates] add constraint fk_ExchangeRates_02 foreign key (toCurrCode) references CurrencyKeys([Key]);
That will define foreign keys for your exchangeRates table.

Answer 2

Don't you just want to do this?
CREATE TABLE [ExchangeRates]
(
[DateTime] [datetime]NOT NULL,
[FromCurrCode] [nchar](3) NOT NULL,
[ToCurrCode] [nchar] (3) NOT NULL,
[Rate] [money] NOT NULL,
Foreign Key([FromCurrCode]) References [CurrencyKeys]([Key]),
Foreign Key([ToCurrCode]) References [CurrencyKeys]([Key])
)
Or are you trying to ask how to maintain primary key values for ExchangeRates table itself. Can you please make yourself clear?

Answer 3

It depends on whether your table could have several rows referring to the same (From,To) pair (to track different exchange rates on different dates)
If that is the case, then the PK could be (FromCurrCode, ToCurrCode, DateTime)
If you are only saving one/the last exchange rate, then (FromCurrCode, ToCurrCode) should be enough.

Answer 4

It appears that you are using natural keys in lieu of surrogate keys.
In order for the ExchangeRates table to use natural keys, you would need to create a composite primary key using FromCurrCode and ToCurrCode
CREATE TABLE [ExchangeRates]
(
[DateTime] [datetime]NOT NULL,
[FromCurrCode] [nchar](3) NOT NULL,
[ToCurrCode] [nchar] (3) NOT NULL,
[Rate] [money] NOT NULL
CONSTRAINT [PK_ExchangeRates] PRIMARY KEY ([FromCurrCode], [ToCurrCode])
)