Won't let me add a Foreign Key into a table in SQL Server

Question

I am tidying up a database and trying to sort all the dependencies out and I keep running into an error when it comes to adding a Foreign Key into a table. I have checked for constraints and can't seem to discover any so I am just wondering where the issue lies.

'PATIENTS' table saved successfully 'Appointments' table - Unable to create relationship 'FK_Appointments_PATIENTS'. The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_Appointments_PATIENTS". The conflict occurred in database "OEPD_PRO", table "dbo.PATIENTS", column 'PatientNumber'.

This is the error which keeps flagging up when I try to save the table. 'PatientNumber' is the Primary Key in the 'PATIENTS' table and I'm trying to add it as a foreign key into the 'Appointments' table.

ALTER TABLE Appointments
ADD CONSTRAINT FK_PatientAppointments
FOREIGN KEY (PatientNumber) REFERENCES PATIENTS(PatientNumber);

I'm very grateful for any help/advice given.

Thanks, KB

Answer 1

Check Appointments for invalid PatientNumber s

select *
from Appointments a 
where not exists (
  select 1
  from Patients p
  where a.PatientNumber = p.PatientNumber
  )

If they need to be removed you can delete them like so:

delete a
from Appointments a 
where not exists (
  select 1
  from Patients p
  where a.PatientNumber = p.PatientNumber
  )

Then try adding your foreign key.

Answer 2

It can be because there are already data in the table that does not match the Foreign Kye values. Suppose I have the values 1,2 and 3 in the Patients.PatientId and the field Appointments.PatientId has a value 4 so that does not meet the constraint rule. So Before adding the constraint make sure that you does not have any such invalid data in the table.