Second generation DBMS’s

Relational Database Model (Second generation DBMS’s)

To overcome the limitation, Edgar F. Codd and others developed the relational data model during the 1970s. This model considered as the second generation DBMS, received widespread commercial accepted during 1980s.

With the relational model, all data are represented in the form of tables. It is based on logical relationship in data, overcame the problems of physical address handling. It is based on mathematical theory and therefor has a solid mathematical foundation. A relatively simple fourth generation language called SQL is used for data retrieval.

• Data Structure: data are store in the form of tabular structure
• Data manipulate: powerful operations are used to manipulate data
• Data integrity: facilities are included to ensure data integrity

Relation: a name two- dimensional table consisting of rows and columns of data. Each relation consists of set of named columns and number of rows (actually a set off tuples).

Employee (Empno, Name, Address)

Empno	Name	Address
1111	Steew	Colombo 5
1115	jone	Colombo 7
1112	Kumara	Colombo 5
1114	Perera	Colombo1

Relational Model Terminologies

Tuple	A row in a relation
Attribute	A  column in a relational
Attribute domain	A set of valid values for an attribute
Degree of a relation	The number of attributes in a relation
Cardinality of a relation	The number of tuples in a relation
Null	Value is not applicable or the value is not known.

Properties of a Relation

• Each relation in a data base must have a unique name.
• An entry at the intersection of each row and columns is atomic or single-valued. (No multi-valued attributes are allowed)
• Each tuple must be unique; no two rows in a relation are identical.  
• Each attribute (or columns) with in a table has unique name.
• The sequence of columns (left to right) is insignificant
• The sequence of rows (top to bottom) is significant