Question

I want a foreign key between 2 tables , so i try it like i always do. Now the issue i’m having is he fails to create , and by the looks of it it fails to create because there is already a key but there isnt.

- Unable to create relationship 
 'FK_tbl_Paramed_RegistratieBehandelingen_Users'.  
  The ALTER TABLE statement conflicted with the 
  FOREIGN KEY constraint "FK_tbl_Paramed_RegistratieBehandelingen_Users". 
  The conflict occurred in database "Nestor_Server", 
  table "dbo.Users", column 'UserID'.

Ive checked if they have the same type , they do(bigint) so don’t get why he won’t create it

Answer 1

It is possible that you have records in RegistratieBehandelingen(Not sure about the table name) which is not present in Users Table.

select * from RegistratieBehandelingen a where UserID IS NULL or
not exists (select 1 from Users b where b.UserID= a.UserID)