Differentiate between primary key constraint and unique constraint. Why is unique constraint preferred over primary key constraint?

A database can be defined as a collection of related data from which users can efficiently retrieve the desired information. A database can be anything from a simple collection of roll numbers, names, addresses and phone numbers of students to a complex collection of sound, images and even video or film clippings. Though databases are generally computerized, instances of non-computerized databases from everyday life can be cited in abundance. A dictionary, a phone book, a collection of recipes and a TV guide are examples of non-computerized databases. The examples of computerized databases include customer files, employee rosters, books catalog, equipment inventories and sales transactions.

A Database Management System (DBMS) is an integrated set of programs used to create and maintain a database. The main objective of a DBMS is to provide a convenient and effective method of defining, storing, retrieving and manipulating the data contained in the database.

To provide a high degree of data independence, the definition or the description of the database structure (structure of each file, the type and storage format of each data item) and various constraints on the data are stored separately in a table. This table is known as the DBMS catalog. The information contained in the catalog is called the metadata (data about data).

The property of DBMS that allows program-data independence is known as data abstraction. Data abstraction allows the database system to provide an abstract view of the data to its users without giving the physical storage and implementation details.

The various cost and risk factors involved in implementing a database system are:

High cost: Installing a new database system may require investment in hardware and software. The DBMS requires more main memory and disk storage. Moreover, DBMS is quite expensive. Therefore, a company needs to consider the overhead cost of implementing a new database system.

Training new personnel: When an organization plans to adopt a database system, it may need to recruit or hire a specialized data administration group, which can coordinate with different user-groups for designing views, establishing recovery procedures and fine tuning the data structures to meet the requirements of the organization. Hiring such professionals is expensive.

Explicit backup and recovery: A shared corporate database must be accurate and available at all times. Therefore, a system using on-line updating requires explicit backup and recovery procedures.

System failure: When a computer system containing the database fails, all users have to wait until the system is functional again. Moreover, if DBMS or the application program fails, a permanent damage may occur to the database.