Introduction to Database

What is a database? A database is a collection of structured data. A database captures an abstract representation of the domain of an application.

• Typically organized as “records” called as entities
• and relationships between records

A DBMS is a (usually complex) piece of software that sits in front of a collection of data, and mediates applications accesses to the data, guaranteeing many properties about the data and the accesses.A data model is a collection of high-level data description constructs that hide many low-level storage details. A DBMS allows a user to define the data to be stored in terms of a data model. Most database management systems today are based on the relational data model.A widely used semantic data model called the entity-relationship (ER) model allows us to pictorially denote entities and the relationships among them.

A description of data in terms of a data model is called a schema.In the relational model, the schema for a relation specifies its name, the name of each field (or attribute or column), and the type of each field.

In addition to the relational data model (which is used in numerous systems, including IBM's DB2, Informix, Oracle, Sybase, Microsoft's Access, FoxBase, Paradox, Tandem, and Teradata), other important data models include ,

• Hierarchical model (e.g., used in IBM's IMS DBMS)
• Network model (e.g., used in IDS and IDMS)
• Object-oriented model (e.g., used in Objectstore and Versant)
• Object-relational model (e.g., used in DBMS products from IBM, Informix, ObjectStore, Oracle, Versant, and others).

The database description consists of a schema at each of these three levels of abstraction:  

• Conceptual Schema : Describes the stored data in terms of the data model of the DBMS.Describes all relations that are stored in the database
• Physical Schema : specifies additional storage details.summarizes how the relations described in the conceptual schema are actually stored on secondary storage devices such as disks.
• External Schema :allows data access to be customized (and authorized) at the level of individual users or groups of users

A data definition language (DDL) is used to define the external and coneeptual schemas (SQL is a well known DDL).The process of arriving at a good physical schema is called physical database design.The process of arriving at a good conceptual schema is called conceptual database esign.