Cardinality

Defines the numerical attributes of the relationship between two entities or entity sets. The three main cardinal relationships are one-to-one (1:1), one-to-many (1:m), and many-many (m:n).

A one-to-one example: would be one student associated with one login name.

A one-to-many example (or many-to-one, depending on the relationship direction): One product line contains multiple products.

Many-to-many example: Students as a group are associated with multiple faculty members (teachers), and faculty members (teachers) in turn are associated with multiple students.

Cardinality views: Cardinality can be shown as look-across or same-side, depending on where the symbols are shown.

Cardinality constraints and obligated (or participation constraint): The minimum or maximum numbers that apply to a relationship