Record based logical models

Record based logical models are used in describing data at the logical and view levels. It is used to specify the overall logical structure of the database.

Record based models are so named because the database is structured in fixed format records of several types. Each record type defines a fixed no. of fields, or attributes, and each field is usually of a fixed length. The three most widely accepted record based data models are the relational, network and hierarchical models.

i) Relational model

The relational model uses a collection of relations(tables) to represent both data and the relationships among those data. Each table has multiple columns, and each column has a unique name.

Customer Name	Address	Account No.	Account No.	Balance
Ram	KTM	A-1	A-1	500
Laxman	Lalitpur	A-2	A-2	700
Bharat	Jhapa	A-3	A-3	900

Fig. A sample relational database

ii)Network model

Data in the network model are represented by collections of records and relationships among data are represented by links, which can be viewed as pointers.

Ram	KTM	0001		A-1	500
Laxman	Lalitpur	0002		A-2	900
Bharat	Janakpur	0003		A-3	6000

Fig. A sample Network database

CustomersName	DustomerStreet	CustomerCity	deposit	Account No	Balance
Customer				Account

Fig. data structure diagram for network data model

iii) Hierarchical model

It is similar to the network model in the sense that data and relationships among data are represented by records and links, respectively. Records are organized as collections of trees rather than arbitrary graphs.

                                                                           Fig. A sample hierarchical database                 

Custom erName	CustomerStreet	CustomerCity	Customer
Account No.	Balance		Account

Fig. Tree structure diagram for hierarchical model

Physical models

Physical data models are used to describe data at the lowest level. There are only few physical data modes in use. Two widely known ones are the unifying model and the frame memory model.

Schemas and instances

Database=description of database + database itself

The overall design of the database is called database schema. Database schema is specified during database design and not expected to change frequently.

Student			Course
Name	Class	Major	Course Name	Duration	Remarks

Fig. Schema Diagram

Database changes over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called instances in the database. It is also called database state or snapshot or current set of occurrences. When database is designed, the database is in empty state with no data. It is in initial state when database is loaded with data. Thus at any point, database has a current state. When any field is added to database, it is called schema evolution.