Tutorial 3
• This tutorial went through how to convert
multiplicity numbering used in UML modelling
to ERD, and vice versa.
• In the exam and assignments, use ERD please!
• We also discussed how to map ERD into
schema using Assignment 2 Q2 as an example.
Multiplicity of Relationship
• At least m, at most n (m .. n)
– Sometimes, you may have (0 .. n),
which means the two entities
does not have to be related all the
time.
• * indicating any number, at least
m is (m .. *).
• * is also a short hand for (0 .. *).
• 1 is a short hand for (1 .. 1).
• Default is 1 to 1.
•
•
•
•
•
m .. n
0 .. n
m .. *
0 .. * (*)
1 .. 1 (1)
ERD
k
e
y
•
•
•
•
• Entities participate at most once in the relationship would
specify key constraints, represented with an arrow.
• Always pointing from the entities to relationships.
• In the case of M-1, it is always from Many table pointing to the
relationship.
One to One
Many to One
Many to Many
Complete (full
participation)
– Complete 1-1
– Complete M-1
– Complete M-M
One
R
One
Many
R
One
Many
R
Many
Entities participate at least once in the relationship
would specify full participation constraints.
One
R
One
Many
R
One
Many
R
Many
Types of relationships - UML
•
•
•
•
One to One
Many to One
Many to Many
Complete (full
participation)
– Complete 1-1
– Complete M-1
– Complete M-M
Entity_A
Attribute1:P
K
Attribute2
….
Entity_B
0 .. 1
relationship
0 .. 1 Attribute1:PK
Attribute2
*
*
0 .. 1
* ….
Entity_A
Attribute1:P
K
Attribute2
….
Entity_B
1
1 .. *
1 .. *
relationship
1
Attribute1:P
K
1 .. 1
1 .. * Attribute2
….
Question – Assignment 2 Q2, Q3
• Draw and Entity-Relationship Diagram for the following scenario, a real
estate firm employs personnel and lists for sale properties that have
owners. The following describes the organisation:
– The firm is made up of several sales offices in various locations. Attributes of
the sales office include an office number (identifier) and a location.
– Each sales office has one or more employees. Each employee has an employee
id (identifier) and a name. Each employee can only be assigned to one sales
office.
– Each sales office has one of its assigned employees designated as the manager
of that office.
– The firm lists properties for sale. Each property has a property id (identifier),
and location information consisting of a street address, the city, state, and
post code.
– Each property is exclusively listed with one sales office. Each sales office may
have many properties listed, including none.
– Each property has one or more owners. The percentage of ownership is an
attribute of the relationship between an owner and the property. Each owner
has an owner id (identifier) and a name. An owner may own one or more
properties.
Attribute or Entity?
Model Answer
Entities participate at least and at most once – exactly once in the relationship are
good candidates for the relationship to merge into as single tables.
Direct Translations – 8 Tables (both entities are relationships)
After considering key and participation constraints – 5 Tables
List relationship
Works-in
relationship
Manages
relationship