ERD Optionality

OOD Inge Powell

ERD Optionality

When we show relationships between tables as well as the cardinality we also need to show the optionality.

Depending on the data we are storing there are several options to choose from.

If using Visio to create a diagram, it takes its initial value from the database store.

ERD Optionality

The cardinality shows what type of relation ship, (One to One or One to Many etc)

But is does not state whether this is a mandatory or optional relationship.

A Mandatory relationship is where there must be at least one matching record in each entity.

An Optional relationship is where there may or may not be a matching record in each entity.

ERD Optionality

The relationship is shown with a connecting line between the entities.

The cardinality is shown by the use of crows feet or lines.

But is does not state whether this is a mandatory or optional relationship?

Entity A Entity B

Entity A Entity B

ERD Optionality

The optionality is shown inside the cardinality.

Entity A Entity B

The optionality is shown inside the cardinality.

Cardinality

Optionality

ERD Optionality

A Mandatory relationship is shown with a vertical line next to the cardinality. (Must be at least one Entity A)

An Optional relationship is shown with a hollow circle next to the cardinality (There maybe 0 to many of Entity B

Entity A

Entity B

ERD Optionality

There are several types. Consider the optionality of the next few examples when combined with cardinality.

At least one entity. One and only one entity.

None or one entity. May be none or one or more than one entity.

Entity Entity

Entity Entity

ERD Optionality

Here are some Mandatory examples..

One Customer has made at least one order.

One Order is for Only One Customer

Customer Order

Customer Order

ERD Optionality

Here are some Optional examples..

One Protected Species record may be for none, one or many kept reptiles.

One Reptile may or may not have a protected species record.

Protected Species

Reptile

Protected Species

Reptile

ERD Optionality

Here are some Mixed examples.

One Category may have none, one or many Products.

One Product will have one and only one Category.

Category Product

Category Product

ERD Optionality

Very rarely there is a optional mandatory relationship. This is usually where there are several tables

concerned. This is a mutually exclusive relationship.

A Bus will have one or more MOT records. A Minibus will have one or more MOT records. An MOT record MUST be for either a bus or a

minibus. It must be for at least One of the entities.

Bus

MOT

Minibus

ERD Optionality

Scenario: A small hire car firm has several cars. The company ensures that the cars they

buy have an initial service. Each car is serviced regularly.Records are kept of each car and each

service.

Car Service

ERD Optionality

In the scenario, there will always be at least one service record for a car.

If a service has been recorded then it must have a matching car record.

This is a Mandatory relationship.

Car Service

Will have at least one

Will have a matching car

Erd Optionality

We add in the optionality.

One Car has one or more Service records. One Service record is for only one Car record. The records might be held like this:

Car Service

CarPK CarID Make Model EngineCC

CarPK CarID Make Model EngineCC

ServicePK ServiceID Date GarageFK CarID

ServicePK ServiceID Date GarageFK CarID

ERD Optionality

Look at all of your relationships in turn.

This is a ‘One to Many’ relationship. Ask yourself, Does Entity A, HAVE to have an

Entity B? If it does, then place a single vertical line next

to the crows feet of Entity B. If it does not, then place a circle by the crows

feet of Entity B.

Entity A Entity B

ERD Optionality

Entity A DOES have to have at least 1 Entity B

Entity A does NOT have to have an Entity B.

Entity A Entity B

Entity A Entity B

ERD Optionality

Lets say you decided that it could be optional. Now look at the reverse relationship.

Ask yourself, Does Entity B, HAVE to have an Entity A?

If it does, then place another vertical line next to the cardinality of Entity A.

If it does not, then place a circle by Entity A.

Entity A Entity B

ERD Optionality

Entity B DOES have to have an Entity A

Entity B does NOT have to have an Entity A.

Entity A Entity B

Entity A Entity B

Erd Optionality

Visio will pick up the relationship types from the database structure.

If it is not showing the optionality correctly, then the issue is in the database relationships.

A One To Many relationship will give an optional link as default.

You must set Referential Integrity to true and correctly set the Join type.

Customer Order????