Entity relationship diagrams show relationships between tables using lines and cardinality notation. Cardinality describes whether a relationship is one-to-one, one-to-many, many-to-one, or many-to-many. Many-to-many relationships cause data problems and are resolved using a link table to create two one-to-many relationships. For example, a books lending relationship between lenders and books would be resolved using a lend records link table.

1. ERD Cardinality
OOD Inge Powell

2. ERD Cardinality
 When
we show relationships between
tables we need to show the cardinality.
 Depending on the data we are storing
there are several options to choose
from.
 If using Visio to create a diagram, it
takes its initial value from the database
store.

3. Erd Cardinality
 The
cardinality relationship is shown with
lines between two entities to indicate a
relationship.
Entity A
 This
Entity B
line denotes that there is a
relationship between the entities.

4. ERD Cardinality
 The
ENDS of these lines denote the cardinality
to that entity.
Entity A
 This
Entity B
is a ‘One to One’ relationship.
 This means that Entity A will relate to only one
of Entity B.
 And Entity B will relate to only one of Entity A

5. Erd Cardinality
 Here
is an example..
Bus
Driver
 One
Driver drives just one Bus.
 One Bus has just one Driver.
 The records might be held like this:
Bus
Bus
PK BusID
PK BusID
Colour
Colour
NoOfSeats
NoOfSeats
FK DriverID
FK DriverID
Driver
Driver
PK DriverID
PK DriverID
Name
Name
Points
Points
FK BusID
FK BusID

6. ERD Cardinality
 The
use of ‘Crows Feet’.
Entity A
 This
Entity B
is a ‘One to Many’ relationship.
 This means that Entity A will relate to many of
Entity B.
 And Entity B will relate to only one of Entity A.

7. Erd Cardinality
 Here
is an example.
Customer
Order
 One
Customer may have many orders.
 One Order is for just one Customer.
 The records might be held like this:
Customer
Customer
PK CustID
PK CustID
Name
Name
Address
Address
TelNo
TelNo
Order
Order
PK OrderNo
PK OrderNo
Date
Date
FK CustID
FK CustID

8. ERD Cardinality
 The
use of ‘Crows Feet’.
Entity A
Entity B
 This
is a ‘Many to One’ relationship.
 This means that Entity A will relate to only one
of Entity B.
 And Entity B will relate to many of Entity A.

(We still tend to refer to as a one to many, just the
other way round from B to A)

9. Erd Cardinality
 Here
is an example.
Product
Supplier
 One
Product will have just one Supplier.
 One Supplier will supply many Products.
 The records might be held like this:
Product
Product
PK ProdID
PK ProdID
PName
PName
Price
Price
Supplier
Supplier
PK SuppID
PK SuppID
Name
Name
Address
Address
TelNo
TelNo

10. ERD Cardinality
 The
use of ‘Crows Feet’.
Entity A
 This
Entity B
is a ‘Many to Many’ relationship.
 This means that Entity A will relate to many of
Entity B.
 And Entity B will relate to many of Entity A.

11. ERD Cardinality
 Many
to Many - This can cause problems with
data, as we can’t pinpoint the link.
Entity A
Entity B
 Imagine
this, a Lender can borrow many
Books. But a Book can be borrowed by more
than one Lender.
Lender
Book

12. ERD Cardinality
 How
do we know who has each individual
book?
Lender
Book
 Also
when we look at the data, how will we
store this?
Lender
Lender
PK LenderID
PK LenderID
Name
Name
Address
Address
FK BookID
FK BookID
FK BookID
FK BookID
FK BookID
Book
Book
PK BookID
PK BookID
FK
FK
Title
Title
Author
Author
LenderID
LenderID

13. ERD Cardinality
 To
manage the many to many relationship, the
data would need to keep growing.
 This
still causes problems,
 Who has the book now?
 This
 We
is silly!!
can resolve this
 By adding a link table.
Book
Book
PK BookID
PK BookID
Title
Title
Author
Author
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID
FK LenderID

14. ERD Cardinality
 To
resolve a many to many relationship.
Entity A
 We
Entity B
resolve it by adding a link table.
Entity A
 This
Entity C
Entity B
means we now have two manageable
one to many relationships instead.

15. ERD Cardinality
 The
resolved many to many relationship.
Lender
LendRecord
Book
 One
lender can have several LendRecords
 One LendRecord is for just one Lender
 One
Book can have several LendRecords
 One LendRecord is for just one Book

16. ERD Cardinality
 Here
is the resolved example.
Lender
LendRecord
Book
 The
data for this system might look something
like this:
Lender
Lender
PK LenderID
PK LenderID
Name
Name
Address
Address
Book
Book
LendRecor
LendRecor
d
d
PK RecordID
PK RecordID
FK BookID
FK BookID
FK LenderID
FK LenderID
PK BookID
PK BookID
Title
Title
Author
Author

17. ERD Cardinality
 The
LendRecord record could be deleted
when the book is returned.
Lender
 Or
LendRecord
Book
fields added to identify the current loan.
Lender
Lender
PK LenderID
PK LenderID
Name
Name
Address
Address
Book
Book
LendRecor
LendRecor
d
d
PK RecordID
PK RecordID
FK BookID
FK BookID
FK LenderID
FK LenderID
OnLoanYN
OnLoanYN
Date
PK BookID
PK BookID
Title
Title
Author
Author