While designing a database is not a difficult task for small and medium sized databases, it is an important one. Bad database design can lead to an inefficient and possibly unreliable database system. Because client applications are built to work on specific parts of a database, and rely on the database design, a bad design can be difficult to revise at a later date. Database design concepts
In designing a database, you plan what things you want to store information about, and what information you will keep about each one. You also determine how these things are related. In the common language of database design, what you are creating during this step is a conceptual database model. Entities and relationships
The distinguishable objects or things that you want to store information about are called entities. The associations between them are called relationships. In the language of database description, you can think of entities as nouns and relationships as verbs. Conceptual models are useful because they make a clean distinction between the entities and relationships. These models hide the details involved in implementing a design in any particular database-management system. They allow you to focus on fundamental database structure. Hence, they also form a common language for the discussion of database design. Entity-relationship diagrams
The main component of a conceptual database model is a diagram that shows the entities and relationships. This diagram is commonly called an entity-relationship diagram. In consequence, many people use the name entity-relationship modeling to refer to the task of creating a conceptual database model. Conceptual database design is a top-down design method. Entities
An entity is the database equivalent of a noun. Distinguishable objects such as employees, order items, departments and products are all examples of entities. In a database, a table represents each entity. The entities...