Relational Database Model

In relational database model, the data is organized into tables(i.e. , rows and columns). These table are called relations. A row in a table represents a relationship among a set of values. Since a table is a collection of such relationships, it is generally referred to using the mathematical term, relation, from which the relational database model derives its name.

Let us see how a sample database can be represented in relational form. The sample database being shown here has three tables(relations): Suppliers, items, shipments.

SuppNo	Supp_name	Status	City
S1	Britannia	10	Delhi
S2	New Bakers	30	Mumbai
S3	Mother Dairy	10	Delhi
S4	Cookz	50	Bangalore
S5	Haldiram	40	Jaipur

Suppliers table

ItemNo	Item_Name	Price
I1	Milk	15.00
I2	Cake	5.00
I3	Bread	9.00
I4	Milk Bread	14.00
I5	Plain biscuit	6.00
I6	Cream biscuit	10.00
I7	Ice Cream	16.00
I8	Cold drink	8.00
I9	Namkeen	15.00

items table

SuppNo	ItemNo	Qty_supplied
S1	I2	10
S1	I3	20
S1	I6	20
S2	I4	20
S2	I5	10
S3	I1	10
S3	I7	10
S4	I8	30
S5	I9	30

shipments table

In the above tables SuppNo, ItemNo, are the primary keys.

• A primary key is the key-field which can uniquely identify a row in a relation.

Notice that here each supplier has unique supplier number, exactly one name, status value and location.. Likewise, we assume that each item has a unique item number, name and price; and also that, at any given time no more than one shipment exists for a given supplier/item combination.

Each of these three tables closely resembles a conventional sequential file, with rows of the table corresponding to records of file and columns corresponding to fields of the records. Each of these tables is actually a special case of construct known in mathematics as a relation.

Rows of relation are generally known as tuples and columns are referred to as attributes.