What do you understand by the term data abstraction?

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.

The attribute that is used to uniquely identify an instance of an entity is known as an

The attributes that are indivisible are known as

In some cases, two or more attribute values are related in such a way that the value of one attribute can be determined from the value of other attributes or related entities. Consider an entity type PERSON and its attributes Date_of_birth and Age. The value of the attribute Age can be determined from the current date and the value of Date_of_birth. Thus, the attribute Age is known as a derived attribute and the attribute Date_of_birth is known as a stored attribute.

The attributes that can have only one value for a given entity are called the

The attributes that are formed by arbitrarily nesting the composite and multivalued attributes are called complex attributes. This is represented by grouping the components of a composite attribute between parentheses ‘()’ and by displaying the multi-valued attributes between braces ‘{}’, and attributes separated by commas. For example, a publisher can have offices at different places, each with different addresses and each office having multiple phones (Figure below).