ENTITY TYPE AND ENTITY SET

Entity Type : An entity type defines a collection of entities that have the same attribute. An entity is an instance of an entity type. All entities having the same set of properties are grouped into an entity type.

Entity sets : An entity set is an collection of similar entities, which share same properties or attributes. Each entity set has a key and each attribute has a domain. The individual entities that constitute a set are said to be extension of the entity set.

ENTITY AND ATTRIBUTES

Entity : An entity is a "thing" in real world, with an independent existence. It may have a physical existence in the real world or it may be an object with a conceptual existence.

Each entity has a set of particular properties that completely describe it, in the context of the mini-world model.

Attributes : The entity properties are called attributes. Attributes are properties used to describe an entity; for example an Employee entity may have a Name, Employee number, Address, Sex, Birth Date etc.

Several types of attributes occur in the ER model. They are;

1. Simple : Each entity has a single atomic value for the attribute.

2. Composite : An attribute may be composed of several components.

3. Multi-valued : An entity may assume multiple values.

OVERALL SYSTEM STRUCTURE

Database systems are partitioned into modules for different functions. Some functions may be provided by the operating system. The components include;

1. File manager : Its manages allocation of disk space and data structures used to represent information on disk.

2. Database manager : The interface between low-level data and application programs and queries.

3. Query processor : Its translates statement in a query language into low-level instruction the database manager understands.

4. DML Precompiler : Converts DML statements embedded in an application program to normal procedure calls in a host language. The precompiler interacts with the query processor.

5. DDL Compiler : Converts DDL statements to a set of tables containing metadata stored in a data dictionary.

6. Data Files : Store the database itself.

7. Data dictionary : Stores information about the structure of the database. It is used heavily. Great emphasis should be placed on developing a good design and efficient implementation of the dictionary.

8. Indices : provide fast access to data items holding particular values.

                                                                                           Database System Structure