Database

Introduction

In the last few years many management and organizations depend on database or databases to make their decisions, so the design and structure of the database be came very important. In this essay I will discuss some of the database concepts.

The Relational Model

The relational database model was conceived by E. F. Codd in 1969, then a researcher at IBM. The model is based on branches of mathematics called set theory and predicate logic. The basic idea behind the relational model is that a database consists of a series of unordered tables (or relations) that can be manipulated using non-procedural operations that return tables. This model was in vast contrast to the more traditional database theories of the time that were much more complicated, less flexible and dependent on the physical storage methods of the data.

The logical view of Data

The relational database model enables us to think about out data logically rather than physically and we design our database base on the entities and tables and their attributes and provide it as simple diagrams to the users.

Keys and Integrity Rules

There is some Rules should be considered when we work with Relational database. These rules determine what is stored in a table, and how the tables are related.

Tables represent "things" in the real world. Each table should represent only one thing. Examples of objects (entities) are customers, employees. Examples of events include orders, appointments, and doctor visits.

Table must contain a unique Key (identifier). Without a unique identifier, it becomes programmatically impossible to uniquely address a row. This called a primary key.

The primary key is an attribute or a combination of attributes that uniquely identifies a given entity (row). It is generally a good idea to pick a primary key that is

• Minimal (has as few columns as possible)

• Stable (rarely changes)

• Simple (familiar to the user)

On other hand there are many keys used on relational database model I will summarize it as the following:

• Super key An attribute (or combination of attributes) that uniquely identifies each entity in a table.

• Candidate key A minimal super key that does not contain a subset of attributes that is itself a super key.

• Primary key A candidate key selected to uniquely identify all other attribute values in any given row. Can not contain null value.

• Secondary key An attribute (or combination of attributes) used strictly for data retrieval purpose.

• Foreign key An attribute (or combination of attributes) in one table whose values must either match the primary key in anther table or be null.

The two types of integrity rules are referential integrity rules and entity integrity rules.

Referential integrity rules dictate that a database does not contain any orphan foreign key values. This means that

• Child rows cannot be added for parent rows that do not exist. In other words, an order cannot be added for a nonexistent customer.

• A