The Entity-Relationship (ER) model was originally proposed by Peter in 1976 as a way to unify the network and relational database views. Simply stated the ER model is a conceptual data model that views the real world as entities and relationships. A basic component of the model is the Entity-Relationship diagram which is used to … Read more
Database design is defined as: “design the logical and physical structure of one or more databases to accommodate the information needs of the users in an organization for a defined set of applications”. The design process roughly follows five steps: 1. Planning and analysis 2. Conceptual design 3. Logical design 4. Physical design 5. Implementation … Read more
A data model is a conceptual representation of the data structures that are required by a database. The data structures include the data objects, the associations between data objects, and the rules which govern operations on the objects. As the name implies, the data model focuses on what data is required and how it should … Read more
Several criteria are normally used to classify DBMSs. The first is the data model on which the DBMS is based. The main data model used in many current commercial DBMSs is the relational data model. The object data model was implemented in some commercial systems but has not had widespread use. Many legacy (older) applications … Read more
Three important characteristics of the database approach are (1) insulation of programs and data (program-data and program-operation independence); (2) support of multiple user views; and (3) use of a catalog to store the database description (schema). In this post we specify an architecture for database systems, called the three-schema architecture, which was proposed to help … Read more
A primary goal of a database system is to retrieve information from and store new information in the database. People who work with a database can be categorized as database users or database administrators. Database Users and User Interfaces There are four different types of database-system users, differentiated by the way they expect to interact … Read more
We can define a data dictionary as a DBMS component that stores the definition of data characteristics and relationships. You may recall that such “data about data” were labeled metadata. The DBMS data dictionary provides the DBMS with its self describing characteristic. In effect, the data dictionary resembles and X-ray of the company’s entire data … Read more
A database system provides a data definition language to specify the database schema and a data manipulation language to express database queries and updates. In practice, the data definition and data manipulation languages are not two separate languages; instead they simply form parts of a single database language, such as the widely used SQL language. … Read more
Underlying the structure of a database is the data model: a collection of conceptual tools for describing data, data relationships, data semantics, and consistency constraints. To illustrate the concept of a data model, we outline two data models in this post: the entity-relationship model and the relational model. Both provide a way to describe the … Read more
Databases change over time as information is inserted and deleted. The collection of information stored in the database at a particular moment is called an instance of the database. The overall design of the database is called the database schema. Schemas are changed infrequently, if at all. The concept of database schemas and instances can … Read more