Question

I’ve got table Visit with columns like VisitID, PatientID, DoctorID etc. and also column PrescriptionID. In table Prescription there are columns PrescriptionID and DrugID (together they are my primary key). One prescription can have many drugs, so there will be something like:

PrescriptionID: 1 DrugID: 38

PrescriptionID: 1 DrugID: 278

PrescriptionID: 1 DrugID: 7

In Visit table there will be inserted ‘1’ value as PrescriptionID. But now I can’t join those two tables with foreign key, because not every visit have prescription, so PrescriptionID can be null and I’ve got error that column must be primary key or must be unique. How can I join those tables in another way?

Answer 1

You seem to be talking about two distinct issues here.

One issue is, you want to allow nulls in a referencing column. That is easy, you just need to define the column as nullable and as a foreign key. What that key should reference is another question, which leads us to the other of the two issues I can see in your question.

The error about the column having to be primary key or unique has to do with the fact that the referencing table uses a single column to reference a compound (consisting of two columns) key. And you can’t reference only part of the key, because a reference must be to a specific single row and your Visit.PrescriptionID value is likely to reference more than one (and rightly so, because the rows in the referenced table aren’t really prescriptions, but prescription items). That is why you are told that the column must be primary key or unique.

Therefore, I suggest changing your schema as follows:

1. Let your Prescription table contain only prescriptions as entities. Even if there are no other attribute than a key, let it be stored in its own table:

   CREATE TABLE Presciption (
     PrescriptionID int
       IDENTITY  /* just an assumption */
       CONSTRAINT PK_Prescription PRIMARY KEY
   );
   

2. Your present Prescription table should be renamed to something like PrescriptionItem or PrescriptionDrug, following your singular noun naming convention. Its PrescriptionID column would be a foreign key referencing Prescription.PrescriptionID, something like this:

   CREATE TABLE PresciptionDrug (
     PrescriptionID int NOT NULL
       CONSTRAINT FK_PrescriptionDrug_Prescription
       FOREIGN KEY REFERENCING Prescription (PrescriptionID),
     DrugID int NOT NULL
       CONSTRAINT FK_PrescriptionDrug_Drug
       FOREIGN KEY REFERENCING Drug (DrugID),
     CONSTRAINT PK_PrescriptionDrug
       PRIMARY KEY (PrescriptionID, DrugID)
   );
   

3. Now you can define the foreign on Visit.PrescriptionID like this:

   ALTER TABLE Visit
   ADD CONSTRAINT FK_Visit_Prescription
       FOREIGN KEY REFERENCING Prescription (PrescriptionID)
   ;
   

   Don’t forget to make sure the column is nullable if you want to make prescriptions optional for visits. (Nullable foreign keys are perfectly fine in SQL Server.)