DATA MODELING - Fundamentals of Database Management Systems - page 23

Cardinality
One-to-One Binary Relationship Figure 2.3 shows three binary relationships of
different cardinalities , representing the maximum number of entities that can be
involved in a particular relationship. Figure 2.3a shows a one-to-one (1-1) binary
relationship, which means that a single occurrence of one entity type can be
associated with a single occurrence of the other entity type and vice versa. A
particular salesperson is assigned to one office. Conversely, a particular office (in
this case they are all private offices!) has just one salesperson assigned to it. Note the
''bar'' or ''one'' symbol on either end of the relationship in the diagram indicating
the maximum one cardinality. The way to read these diagrams is to start at one
entity, read the relationship on the connecting line, pick up the cardinality on the
other side of the line near the second entity , and then finally reach the other entity.
Thus, Figure 2.3a, reading from left to right, says, ''A salesperson works in one
(really at most one, since it is a maximum) office.'' The bar or one symbol involved

SALESPERSON
PK

OFFICE

Salesperson
Number
Salesperson
Name
Commission
Percentage
Year of Hire

PK Office

Number
Telephone
Size

Works in
Occupied by

One
Salesperson

One
Office

(a.) One-to-one (1-1) binary relationship

SALESPERSON
PK

CUSTOMER
PK Salesperson

Number
Salesperson
Name
Commission
Percentage
Year of Hire

Customer
Number
Costomer
Name
HQ City

Sells to
Buys from

One
Salesperson

Many
Customers

(b.) One-to-many (1-M) binary relationship

SALESPERSON
PK

PRODUCT

Salesperson
Number
Salesperson
Name
Commission
Percentage
Year of Hire

PK Product

Number
Product
Name
Unit Price

Sells
Sold by

Many
Products

Many
Salespersons

(c.) Many-to-many (M-M) binary relationship

FIGURE2.3
Binary relationships with cardinalities