The first database systems were based on the network and hierarchical models. A database can be defined as a collection of non-redundant data which can be shared by different application systems. A database implies separation of physical storage from use of the data by an application program to achieve program/data independence. Using a database system, the user or programmer or application specialist need not know the details of how the data are stored and such details are usually "transparent" to the user. .
These are covered briefly in appendices in the text. The relational model was first proposed by E.F. Codd in 1970 and the first such systems were developed in 1970s. The relational model is now the dominant model for commercial data processing applications. The relational model can be used in both conceptual and logical database design. The basic structure in the model is a table .Tables consists of rows and columns. Relationships in the relational model are represented implicitly through common attributes between different relations.
The relational model consist of a relational structure, a set of integrity rules, and data manipulation operations. The relational structure is based on the representation of data in the form of tables. A table contains rows and columns, with each row representing an individual record, and each column representing a field for each record. Tables are related via indirect indexes of primary and foreign keys. The operations that are performed on these tables in order to store, manipulate and access this data include union, intersection, join, division, restriction, projection, assignment, difference, and product. How do you know if the relational model best fits your intended application? An application that requires on-line transaction processing (OLTP) where multiple files are updated simultaneously could benefit from the table structure of the relational model. The relational model provides the ability to quickly insert data into tables. However, when it comes to querying--getting data out of the database--the relational model can be slower because it doesn't support direct access in multiple joins that are possible with the network model. An RDBMS-based application requires the traversal of indexes to get at related data in other files and this requires additional disk accesses and CPU cycles. The more tables involved and the greater the volume of records in these tables the more time is spent in accessing disk storage to get a result set. But while the relational model has the power to meet heavy-duty OLTP needs--get data into the database--it is also an excellent choice for simple databases and for an inexperienced database designer. The relational concept is easy to grasp, and data is easy to understand and manipulate using columns and rows. The ability to easily change the database is a key feature of the relational model. Although planning is a critical step to making any change to the database, with an RDBMS it is easier to change column definitions and add new columns to tables. Tables can be added or deleted without having to restructure the database. ANSI SQL is a standard database manipulation language that is implemented by most of the RDBMS products on the market. Standard SQL provides a common interface for data definition, user access, and database manipulation. SQL has played a critical role as a standard for relational database usage because corporations need only learn one language for multiple database platforms. Even though SQL is a standard that is 99% identical across the board, many unique extensions and features have been added by the database vendors to differentiate their products.
The strength of Relational Model.
The relational model consists of three components:
1.A Structural component -- a set of TABLES (also called RELATIONS). 2.MANIPULATIVE component consisting of a set of high-level operations which act upon and produce...