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Reference
Relationship notation
There are also E-R diagrams that use
“rhomboids” to represent “relationships”
and show “one-to-many” relationships
as “1-to-*,” “1-to-n,” or “1-to-m.”
Customer
n
*
Order
Symbol
Name
Product
Typical modeling techniques
The typical modeling techniques are summarized below.
(1)E-R diagram
An “E-R diagram” expresses the relationship between data using “entities” and “relationships.” Entities and relationships have several characteristics called “attributes.”
There are three types of relationships, namely one-to-one, one-to-many and
many-to-many.
Entity
Entity
Relationship
Attribute
Entity
Entity
(Relationship)
(Relationship)
(Relationship)
Entity
One-to-one
Entity
One-to-many
Entity
Many-to-many
① Entities
are represented by rectangles.
name of the entity is written inside the rectangle.
③ The relationships between entities are represented by straight lines or
arrows. The name of the relationship is written beside the line in parenthesis.
④ “One-to-one” relationships are represented by straight lines.
“One-to-many” relationships are represented by single-headed arrows
pointing to multiple entities.
“Many-to-many” relationships are represented by double-headed arrows.
② The
Example
E-R diagram of the relationship between customers, orders, and
products
Customer
Order
Product
Customer number
Customer number
Product number
Customer name
Product number
Product name
Volume
Unit price
Attributes
There is a one-to-many relationship between “customer” and “orders,”
which implies that a single customer has placed several orders.
There is also a one-to-many relationship between “product” and “orders,”
which implies that multiple orders have been placed for a single type of
product.
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