5.2.2: One-To-One

If you drag a PK field of one table to another table, and if the FK has unique values (a unique index exists for it) then you are creating a one-to-one relationship. For each row in the first table there can be at most one related row in the other table; a row in the referenced table has a PK value that equals the FK value in at most one row of the referencing table.