DBMS: Getting Data Together

The Hierarchy of Data

• Field – a group of one or more characters that has a specific meaning
• The smallest meaningful unit of data
• Describes one characteristic of a person, place, or thing
• Record – the set of fields containing data about a person, place, or thing
• File – a collection of related records

File System Approach

• Each application had its own file

Data was not shared among application

• Resulted in a great deal of data redundancy, the repetition of the same data value

Increased the risk of inaccurate data
Increased the amount of storage space needed.

Database Approach

A collection of related tables

• In database technology, a file is called a table
• Each entity is stored in a separate table
• Tables are linked by a relationship between primary and foreign keys
  

Primary Key

A field that uniquely identifies a record

• SalesID can be a primary key for the Salesperson table
• Once a salesID appear in the table, no other salesperson can have that ID

Foreign Key

A field in one table that is a primary key in another table

• SalesID can be used in the Customer table to identify the salesperson who serves that customer
• The same SalesID can appear in many customer records (a salesperson can serve many customers)

DATABASE MODEL

• Define the way a database organizes data
• Four main models

1. Hierarchical
2. Network
3. Relational
4. Object-oriented

Hierarchical Model
1. Arranges data in hierarchical "parent-child" relationship

• Each parent record can have many child records
• Each child record has only one parent record

2. Complex and inflexible


Network Model
1. Arranges data in complex network of "parent-child" relationship

• Each parent record can have many child records
• Each child record can have many parent records

2. Complex and inflexible

Relational Model
1. Data organized in table format

• Columns represent fields
• Row represent record

2. Tables related by primary/foreign key relationship
3. Most current database development uses this model



Object-oriented Model
1. Designed to deal with complex data types
2. Focuses on the object

• An object represents an entity
• Represents data about that entity and the types of operations that change that entity