Implementing a “Both, Either-or, but Not Null” Requirement in a Database

Question

I have requirement for a web app that states that a user should be able to either upload an instruction document(s) (.pdf, .doc, .txt) or provide text for the instructions. The

Answer 1

I think that this cannot be done with Declarative Referential Integrity alone - not if your design has these 3 separate tables.

You'll have to ensure that all Insert/Delete/Update operations are done within transactions (stored procedures) that enforce such a requirement - so no row is ever inserted or left in table Instruction without a relative row in either one of the 2 other tables.

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If you don't mind having nullable fields, you could merge the 3 tables into one and use a CHECK constraint:

CREATE TABLE Instruction
( InstructionID INT          NOT NULL
, Text          VARCHAR(255) NULL
, Filepath      VARCHAR(255) NULL
, PRIMARY KEY (InstructionID)
, CONSTRAINT Instruction_has_either_text_or_document 
    CHECK (Text IS NOT NULL OR FilePath IS NOT NULL)
) ;