Assume two entities, A and B.
A 1:1 relationship exists between A and B when each occurrence of entity A is related to at most one occurrence of entity B, while each occurrence of entity B is related to at most one occurrence of entity A.
1:1 relationships are not commonly seen in real world situations.
1:1 relationships are graphed as follows:
Modeling 1:1 Relationships
1:1 relationships are modeled by placing the primary key of entity A as a foreign key component of entity B with no duplicates allowed. Because the relationship is symmetrical, you could just as well place the primary key of entity B as a foreign key component of entity A.
Guideline for Placing the Foreign Key
Place the foreign key in whichever entity minimizes or eliminates the possibility of nulls.
Example
Because of the symmetry of 1:1 relationships, the following entity pairs both model the same relationship: