Assume two entities, A and B.
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.
Somewhat more formally, Date defines
1:1 relationships as follows: “Let A and B be sets, not necessarily distinct. Then a one-to-one correspondence from A to B is a rule that pairs each element of A with exactly one element of B and each element of B with exactly one element of A. Equivalently, we might just say the one‑to-one correspondence is that pairing itself”
(2006, page 14).
1:1 relationships are not commonly seen in real world situations.
1:1 relationships are graphed as follows: