Overview of the (E)ER Model Notation
Based on the book by Saake, Sattler, Heuer “Datenbanken: Konzepte und Sprachen” 5.Auflage, mitp-Verlag, 2013
Basic Notation Elements
Entity
Entity types are the units of information represented by a database.
Example: A database stores information about customers and products.
Relation
Relationship types describe relationships between entities.
Example: Customers buy products.
Attribute
Attributes represent properties of entities or relationships.
Example: A customer has a name.
Key
Attribute
Primary Keys (Key Attributes) are attributes that uniquely identify an
entity of an entity type.
Example: The customer ID uniquely identifies each customer.
Customer
CustNo
Address
Name
Phone
Attribute
Customer
CustNo
Multi-Value Attributes may be specified multiple times.
Example: A customer is able to have multiple phone numbers.
City
Address
Name
Optional Attributes represent properties of entities or relationships that
may exist, but don’t need to.
Example: A customer may specify his phone number, but he doesn’t have
to.
ZIP
Street
Structured Attributes consist of multiple sub-attributes.
Example: The customer address consists of street, zip code and city.
Phone
Net Price
Derived Attributes can be derived from other attributes.
Example: The retail price of a product is calculated from the net price
and additional taxes.
Authors: David Broneske, Sebastian Dorok, Florian Koch
Depending or Weak Entities are functionally determined by another entity. The dependent entity
type doesn’t have any attributes that may be used to uniquely identify an entity of this type. A primary
key, or respectively a set of attributes forming one, always consists of the key attributes of the entity
type it depends on, too.
Example: A vintage always belongs to a wine, since it has a specific residual sweetness. If the wine
doesn’t exists, the vintage doesn’t exist, either. But, since there are multiple wines with a vintage in the
same year, only the combination of wine name and vintage year are able to uniquely identify a vintage.
Alternative
Vintage Year
N
Belongs To
1
Year
Wine
Year
Name
Res.
Sugar
Belongs To
Vintage Year
Wine
Color
Name
Res.
Sugar
Color
IS-Relationships, also called specialization-/generalization-relationships, semantically correspond to injective functional relationships.
Example: A sparkling wine is a special wine. Through this definition the sparkling wine inherits the
attributes name and color of a wine.
Alternative
Manufact.
Method
WName
Sparkling
Wine
Color
Manufact.
Method
WName
Sparkling
Wine
Wine
IS A
Color
Wine
Cardinalities
Cardinalities describe, how often entities of an entity type may have a relation to another entity.
Example: A specific product is delivered by a specific supplier. Therefore, each product has a relationship
to exactly one supplier. On the other hand, a supplier may offer multiple or no products at all. In the
course of this, a supplier can have a relationship with multiple products.
[min, max] notation: Minimal and maximum quantity is specified.
Beispiel
E1
[min1, max1]
R
[min2, max2]
[minn, maxn]
...
En
Product
[1,1]
delivered
by
[0,*]
Supplier
E2
Authors: David Broneske, Sebastian Dorok, Florian Koch
Alternative representation: Note the “reversed” assignment of the cardinalities!
Product
N
1
delivered
by
Supplier
Enhanced/Extended Notation Elements
Alternative
Specialization
I
I
O
I
O
O
Partitioning
O1
I
On
O1
I
On
O1
I
d
On
Total Partitioning
I
=
O1
I
On
On
O1
I
O1
d
On
Multiple Specialization
O1
I
I1
…
In
On
O1
I
On
O1
Generalization
O
I
On
O1
I
I1
o
U
On
O
In
Authors: David Broneske, Sebastian Dorok, Florian Koch