Question

So, I’ve got a table created like so:

create table CharacterSavingThrow
(
    CharacterCode int not null,
    constraint FK_CharacterSavingThrowCharacterID foreign key (CharacterCode) references Character(CharacterCode),
    FortitudeSaveCode int not null,
    constraint FK_CharacterSavingThrowFortitudeSaveCode foreign key (FortitudeSaveCode) references SavingThrow(SavingThrowCode),
    ReflexSaveCode int not null,
    constraint FK_CharacterSavingThrowReflexSaveCode foreign key (ReflexSaveCode) references SavingThrow(SavingThrowCode),
    WillSaveCode int not null,
    constraint FK_CharacterSavingThrowWillSaveCode foreign key (WillSaveCode) references SavingThrow(SavingThrowCode),
    constraint PK_CharacterSavingThrow primary key clustered (CharacterCode, FortitudeSaveCode, ReflexSaveCode, WilSaveCode)

)

I need to know how I would reference the primary key of this table from another table’s constraint? Seems like a pretty simple question, either it’s possible or not, right? Thanks for your guys’s help!

Answer 1

Yes – totally easy – you just have to specify the complete compound index, e.g. your other table also needs to have those four columns that make up the PK here, and then the FK constraint would be:

ALTER TABLE dbo.YourOtherTable
ADD CONSTRAINT FK_YourOtherTable_CharacterSavingThrow
FOREIGN KEY(CharacterCode, FortitudeSaveCode, ReflexSaveCode, WilSaveCode)
REFERENCES dbo.CharacterSavingThrow(CharacterCode, FortitudeSaveCode, ReflexSaveCode, WilSaveCode)

The point is: if you have a compound primary key (made up of more than one column), any other table wanting to reference that table also must have all those columns and use all those columns for the FK relationship.

Also, if you’re writing queries that would join those two tables – you would have to use all columns contained in the compound PK for your joins.

That’s one of the main drawbacks of using four columns as a PK – it makes FK relationships and JOIN queries awfully cumbersome and really annoying to write and use. For that reason, in such a case, I would probably opt to use a separate surrogate key in the table – e.g. introduce a new INT IDENTITY on your dbo.CharacterSavingThrow table to act as primary key, that would make it a lot easier to reference that table and write JOIN queries that use that table.