Specify foreign key on one column and the value of another column

Question

I have a table ASSETS that has a structure as it is shown below :

----------------------------------------------------
ID (PK) | DESCRIPTION | TYPE | Do- | Do+ | Dx- | Dx+
----------------------------------------------------

TYPE column has a foreign key, possible values are SECURITY or CURRENCY (i.e. FX), also I have two more tables : CURRENCIES (for example, EUR, RUB or USD) :

--------------------------------------------------------
ID (PK)| FROM (FK ASSETS.ID) | TO (FK ASSETS.ID) | VALUE
--------------------------------------------------------

and SECURITIES (for example, MTS, GAZP or VTB) :

----------------------------------------------------------
ID (PK)(FK ASSETS.ID)| CURRENCY (PK)(FK ASSETS.ID) | VALUE
----------------------------------------------------------

How I can make a constraint, that not only acts like foreign key in CURRENCIES.FROM, CURRENCIES.TO and SECURITIES.CURRENCY,but also checks if referring ASSETS.TYPE is CURRENCY, and in SECURITIES also checks if referring ASSETS.TYPE for SECURITIES.ID is SECURITY?

I guess I can write triggers to check ASSETS.TYPE value, but I am searching for another solution right now (if it is possible, of course).

If there are better ways to do the things a want (as a better database design), please, share your ideas.

P.S. I guess it is quite a common problem, so if there are articles about it or similar questions asked on this network or some general-case-solutions, feel free to share.

Answer 1

Answer to your original question is to use an additional CHECK constraint like :

CREATE TABLE CURRENCIES (
   ...
   CONSTRAINT c_asset_from CHECK(exists(select 1 from ASSETS a where a.id = from and a.type = 'CURRENCY'))
);

And similar constraion for TO field and in SECURITIES for CURRENCY field.
But I think your new design, with separate FK for security and currency, is better design.

Answer 2

IMO technically the design could be criticized in two categories:

• Having a dual-purpose foreign key in Asset table called type (Polymorphic Association anti-pattern).
  That will violating first normal form (atomic issue), loosing referential integrity.
  A solution could be simplification of the relationship by inheritance.
  Having a base table for Currency and Security tables called Money,containing shared properties of them, like name.
  primary key of Money table will be primary key of Currency and Security tables.
  Having foreign key of Money inside Asset will be the solution.
• Using surrogate identifier on Asset tables, that will result losing business logic in schema design.
  I will prefer haveing composite primary key in Asset Table PK{ID, TYPE(money fk)}.
  Then having check constraints on CURRENCIES and SECURITIES will solve the problem.
  CURRENCIES_chk {FK.CURRENCY = FK_TO.Money && FK.CURRENCY = FK_FROM.Money} SECURITIES_chk {FK.SECURITY = FK.Money}
  
  

Answer 3

You can do that declaratively by changing the design of your keys and using identifying relationships.

Here is the blueprint:

Look how ASSET.ASSET_TYPE is propagated through both "branches", only to be merged in the SECURITY.ASSET_TYPE.

Since SECURITY.ASSET_TYPE is just one field, one SECURITY row can never connect to multiple asset types. To say it slightly differently: if ASSET and CURRENCY are connected to the same SECURITY, they must have the same ASSET_TYPE.

In addition to that, CURRENCY can never point to ASSETs of different type.

You can bring back your old surrogate keys (and other fields) into this model as necessary.

---

That being said, generating ASSET_NO presents some challenges.

• You can just use auto-incrementing mechanism built-into your DBMS, but that would leave "holes" (i.e. two different asset types will never use the same integer, even though they technically can).
• Or you can find the next value manually, but you'll have to handle concurrency in that case (either serialize insertions through locking, or retry insertion in case concurrent transaction tried the same value).

Answer 4

You could use checks for this. Do you want to hardcode these values?

CREATE TABLE Persons
(
    P_Id int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Address varchar(255),
    City varchar(255),
    CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
)

Source: W3schools

And using firebird might require different syntax. Take a look at: Firebird reference