Constraints

 

Constraints

ž Once we have identified entities and relationships, we need to consider constraints

ž Constraints help to maintain the integrity and accuracy of the DB

ž There are two types of constraints used in the ER model, cardinality ratios and existence dependencies (aka participation constraints)

ž Cardinality ratios express the number of entities to which another entity can be associated via a relationship set

ž For binary relationships, there are only four possible cardinality ratios

 

 

Constraints in ER modeling

* role – the function an entity plays in a relationship

* existence constraint (existence dependency) – weak entity

* exclusion constraint – restricts an entity to be related to only of several other

* entities at a given point in time

- mandatory/optional

- specifies lower bound of connectivity of entity instances

- participating in a relationship as 1 or 0

* uniqueness constraint – one-to-one functional dependency among key attributes in a relationship: binary, ternary, or higher n-ary

ER diagram relationsER diagram notation

 

 

 

 

 

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