You are advised to visit posts "What is RDBMS" Part I & II as the following text may contain some references to them!
Database designing is considered as one of the important phases of the database based applications. The database designers must interact with the users of the database application to understand the need of application and represent them in a high-level fashion that can be understood by the users and then translate the requirements into lower levels of the design. A high level data model, particularly E-R model, helps to provide a conceptual framework to represent data requirements of the users and database structure that fulfills these requirements.
- Initial phase of the database design is to characterise fully the data needs of the prospective database users. The outcome of this phase is a specification of user requirements.
- Next, the designer chooses a data model and, by applying the concepts of the chosen data model, translates these requirements into a conceptual "schema" (structure or layout) of the database. The "E-R model" is typically used to represent conceptual design. Conceptual design phase results in the creation of an entity-relationship diagram that provides graphical representation of the schema.
- Next, specification of functional requirements evolve from the conceptual model. Users describe the kinds of operations (say transactions) that will be performed on the data. The access patterns are identified and the designer can review the schema to ensure that it meets those functional requirements.
- Last is the implementation of the abstract data model drawn so far which proceeds in two final design sub-phases –
- The logical design phase typically consists of mapping the conceptual schema defined using the E-R model into a relational schema. (Assuming that the relational data model is typically used in the implementation.)
- In the final Physical design phase the physical features like files organization, internal storage structures etc. are evolved.
- Redundancy – Unnecessary repetition of the information is called as redundancy. Ideally information should appear in exactly one place.
- Incompleteness – Restriction of the database design to not allow insertion of the parent fact for non-availability of the child fact may give rise to incompleteness. Parent fact registration in the database should be independent of the availability of the child fact. For example not able to insert the information of the subject being taught in an institute for no students opting for the subject offered. (from the example of post "What is RDBMS? Part I").
Entity Sets :
An entity set is a set of entities of the same type that share the same properties or attributes.
The individual entities that constitute a set are said to be the extension of the entity set. For example "Customer" is an entity set while "every individual bank customer" is an extension of the entity set customer.
An entity is represented by a set of attributes. Attributes are descriptive properties possessed by each member of an entity set.
Database stores similar information concerning each entity in the entity set; however each entity may have its own value for each attribute.
A database includes a collection of entity sets, each of which contains any number of entities of the same type.
Relationship Sets :