Constraint (E-R)

An E-R enterprise schema may define certain constraints to which the contents of a database must conform.

Mapping cardinality

Mapping cardinality expresses the number of entities to which another entity can be associated via a relationship set. It is most useful in describing binary relationship sets.

For a binary relationship set $R$ between entity sets $A$ and $B$, the mapping cardinality must be one of the following:

• One-to-one
• One-to-many
• Many-to-one
• Many-to-many

One-to-one

An entity in $A$ is associated with at most one entity in $B$, and vice versa.

One-to-many

An entity in $A$ is associated with any number (zero or more) of entities in $B$. An entity in $B$ can be associated with at most one entity in $A$.

Many-to-one

An entity in $A$ can be associated with at most one entity in $B$. An entity in $B$ is associated with any number (zero or more) of entities in $A$.

Many-to-many

An entity in $A$ is associated with any number (zero or more) of entities in $B$, and vice versa.

Participation constraints

Let $E$ be an entity set and $R$ be a relationship set.

• Total participation
  The participation of $E$ in $R$ is said to be total if every entity in $E$ participates in at least one relationship of $R$.

• Partial participation
  The participation of $E$ in $R$ is said to be partial if only some entities in $E$ participate in relationships in $R$.