One-to-one (data model)

In systems analysis, a one-to-one relationship is a type of cardinality that refers to the relationship between two entities (see also entity–relationship model) A and B in which one element of A may only be linked to one element of B, and vice versa.

In mathematical terms, there exists a bijective function from A to B.

In a relational database, a one-to-one relationship exists when one row in a table may be linked with only one row in another table and vice versa.

A list of mothers and their children may happen to describe mothers with only one child, in which case one row of the mothers table will refer to only one row of the children table and vice versa.

The real-world relationship that the data models is not one-to-one, because mothers may have more than one child, thus forming a one-to-many relationship.