3.1 Relational Databases: Terminology

3. The Relational Model

Databases: Case Example

Ord_Aug

Ord #	OrdDate	Cust#
101	02-08-94	002
102	11-08-94	003
103	21-08-94	003
104	28-08-94	002
105	30-08-94	005

Ord_Items

Ord #	Item #	Qty
101	HW1	100
101	HW3	50
101	SW1	150
102	HW2	10
103	HW3	50
104	HW2	25
104	HW3	100
105	SW1	100

Items

Item #	Descr	Price
HW1	Power Supply	4000
HW2	101-Keyboard	2000
HW3	Mouse	800
SW1	MS-DOS 6.0	5000
SW2	MS-Word 6.0	8000

Customers

Ord #	OrdDate	Cust#
101	02-08-94	002
102	11-08-94	003
103	21-08-94	003
104	28-08-94	002
105	30-08-94	005

Term	Meaning	Eg. from the given Case Example
Relation	A table	Ord_Aug, Customers, Items etc.
Tuple	A row or a record in a relation.	A row from Customers relation is a Customer tuple.
Attribute	A field or a column in a relation.	Ord_Date, Item#, CustName etc.
Cardinality of a relation	The number of tuples in a relation.	Cardinality of Ord_Items relation is 8
Degree of a relation	The number of attributes in a relation.	Degree of Customers relation is 3.
Domain of an attribute	The set of all values that can be taken by the attribute.	Domain of Qty in Ord_Items is the set of all values which can represent quantity of an ordered item.
Primary Key of a relation	An attribute or a combination of attributes that uniquely defines each tuple in a relation.	Primary Key of Customers relation is Cust#. Ord# and Item# combination forms the primary Key of Ord_Items
Foreign Key	An attribute or a combination of attributes in one relation R1 which indicates the relationship of R1 with another relation R2. The foreign key attributes in R1 must contain values matching with those of the values in R2	Cust# in Ord_Aug relation is a foreign key creating reference from Ord_Aug to Customers. This is required to indicate the relationship between Orders in Ord_Aug and Customers. Ord# and Item# in Ord_Items are foreign keys creating references from Ord_Items to Ord_Aug and Items respectively.

3. The Relational Model