Entity Relationship Diagrams

Entities

• An entity is something about which the system/business needs to store data.

• Entities are drawn as rectangles

• The rectangle represents all instances of the named entity

• Important to distinguish between an entity and its instances

Attributes

• Entity is something about which we want to store data

• Specific items of data we want to store are data attributes

• To record attributes of the entity we list them within the entity shape

Example

Student

.Last name

.First name

Address

.Street address

.Town

.County

.Post code

Relationships

• Entities and attributes do not occur in isolation.

• The things they represent interact with and impact one another to support the business

• A relationship is a natural business association that exists between one or more entities

Example

• A current STUDENT is ENROLLED ON one or more SUBJECTS

• A SUBJECT is STUDIED BY none (zero), one or more students

STUDENTSUBJECT

Is enrolled on

Is studied by

Verb phrases are used for relationships

Cardinality

The previous example shows the complexity or degree of each relationship

• Must there exist an instance of STUDENT for each instance of SUBJECT – No

• Must there exist an instance of SUBJECT for each instance of STUDENT – Yes

• How many instances of STUDY can exist for each instance of STUDENT – Many

• How many instances of STUDENT can exist for each instance of STUDY - Many

Cardinality continued

• Cardinality defines the minimum and maximum number of occurrences of one entity that may be related to a single occurrence of the other entity

• All relationships are bidirectional therefore cardinality must be defined in both directions for every relationship

Cardinality symbols

• Cardinality interpretation

Exactly one (one and only one)

Minimum instances 1

Maximum instances 1

or

Cardinality

• Cardinality interpretation

Zero or one

Minimum instances 0

Maximum instances 1

Cardinality

• Cardinality interpretation

One or more

Minimum instances 1

Maximum instances many (>1)

Cardinality

• Cardinality interpretation

Zero, one or more

Minimum instances 0

Maximum instances many (>1)

Cardinality

• Cardinality interpretation

More than one

Minimum instances >1

Maximum instances >1

Cardinality

Cardinality tells us the following rules about the data we want to store:

• When we insert a STUDENT instance in the database we must link that student to at least one instance of SUBJECT

• A STUDENT can study more than one SUBJECT and we must be able to store data that includes all SUBJECT(s) for a given STUDENT

Cardinality

• We must insert a SUBJECT before we can link (associate) STUDENTS to that SUBJECT. This is why SUBJECT can have zero students – no students have yet enrolled on that subject.

• Once a SUBJECT has been inserted into the database, we can link (associate) many STUDENTS with that SUBJECT