Strength of Relational Model

Introduction

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 whole tables.

3. A SET OF RULES for maintaining the INTEGRITY of the database.

The terminology associated with relational database theory originates from the branch of mathematics called set theory although there are widely used synonyms for these precise, mathematical terms. Data structures are composed of two components which represent a model of the situation being considered. These are (i) ENTITY TYPES - i.e. data group types, and (ii) the RELATIONSHIPS between the entity types. Entity types are represented by RELATIONS or BASE TABLES. These two terms are interchangeable - a RELATION is the mathematical term for a TABLE. A base table is loosely defined as an un-ordered collection of zero, one or more TUPLES (ROWS) each of which consists of one or more un-ordered ATTRIBUTES (COLUMNS). All tuples are made up of exactly the same set of attributes.

For the remainder of this discussion we shall use the more widely known terminology:

* TABLE for RELATION

* ROW for TUPLE

* COLUMN for ATTRIBUTE

* Each column is drawn from a DOMAIN, that is, a set of values from which the actual values are taken (e.g. a set of car model names). More than one column in a table may draw its values from the same domain.

* A column entry in any row is SINGLE-VALUED, i.e. it contains exactly one item only (e.g. a surname). Repeating groups, i.e. columns which contain sets of values rather than a single value, not allowed.

* Each row of a table is uniquely identified by a PRIMARY KEY composed of one or more columns. This implies that a table may not contain duplicate rows.

* Note that, in general, a column, or group of columns, that uniquely identifies a row in a table is called a CANDIDATE KEY. There may be more than one candidate key for a particular table; one of these will be chosen as the primary key.

* The ENTITY INTEGRITY RULE of the model states that no component of the primary key may contain a NULL value.

* A column, or combination of columns, that matches the primary key of another table is called a FOREIGN KEY.

* The REFERENTIAL INTEGRITY RULE of the model states that, for every foreign key value in a table there must be a corresponding primary key value in another table in the database.

* Only two kinds of table may be defined in a SQL schema; BASE TABLES and VIEWS. These are called NAMED RELATIONS. Other