Thursday, December 31, 2009

Happy New Year 2010

Wishing you and your family a Very Happy New Year 2010.

Bagalamukhi Painting

Wednesday, December 30, 2009

Entity, attribute, attribute value, relationship instance, composite attribute, multivalued attribute, derived attribute, complex attribute,.......

Entity: An entity is a thing in the real world. It may be an object with a physical existence or an object with a conceptual existence. For example, a man, a building, a company, a school etc.

Attribute: Attribute is a property of an object or an entity. For example, a car has a color, a brand name, a model number, an owner’s name, type etc.

Attribute value: Attribute value is the real data of a particular entity for each of its attributes. In other word, associated with each real world entities are certain attributes that describe that entity; value of these attributes for any entity is called attribute value. For example, attribute value of first_name of attribute of student_name can be Gyanendra.

Relationship instance: Each relationship instance ri in R is an association of entities, where the association includes exactly one entity from each participating entity type. Each such relationship instance ri represent the fact that the entities participating in ri are related in some way in the corresponding miniworld situation. For example, in relationship type WORKS_FOR associates one EMPLOYEE and DEPARTMENT, which associates each employee with the department for which the employee works. Each relationship instance in the relationship set WORKS_FOR associates one EMPLOYEE and one DEPARTMENT.

Composite attribute: Composite attribute is an attribute that can be divided into smaller subparts, which represent more basic attributes with dependent meanings, is called a composite attribute. For example, the Address attribute consists of several domains such as house number, street number, city, country, etc.

Multivalued attribute: A multi-valued attribute can have more than one value at one time. For example, address of a person is a multi-valued attribute since a person can have more than one address such as Present and Permanent address. Upper and lower bounds may be placed on the number of values in a multi-valued attribute. For example, a bank may limit the number of addresses recorded for a single customer to two.

Derived attribute: If an attribute’s value can be determined from the values of other attributes, then the attribute is derivable, and is said to be a derived attribute. Example: consider attributes for an employee: birth date, current age; here, age is derivable by subtracting the birth date from the current date.

Complex attribute: Composite and multivalued attribute can be nested arbitrarily. Arbitrary nesting can be represented by grouping components of a composite attribute between parenthesis and separating the components with commas and by displaying multivalued attributes between braces. Such attributes are called composite attributes. For example, if a person has more than one address and each residence has multiple phones and address_phone attribute can be specifies as:
{AddressPhone( (Phone{AreaCode,PhoneNumber)},Address(StreetAddress(Number, Street, ApartmentNumber), City, State, Zip) ) }

Key attribute: Each real world entity is unique in itself. There are certain attributes whose value is different for all similar type of entities. Those attributes are called key attributes. These attributes are used to specify uniqueness constraint in relation. For example, a house has a registration number. This is a key of all entity of house.

Value set (domain): There is a range of values from which a particular attribute can take value for a attribute of a real world entity. For example, salary attribute of an employee must have value, let, from $2000 to $12000, and then all integers in range $2000 to $12000 are domain of attribute salary.

Friday, December 25, 2009

The various cases where use of a null value would be appropriate.

When there is an entity that has no value for its attribute, then we use a null value. A null value is not 0, but it is nothing value. One example is most Korean names have no middle name. If there is a name attribute with first name, middle and last name, a special value null should be given. Another example is, asylum applicant’s has no nationality. When they fill an immigration form for the particular country when they visit there, they do not write country’s name. So, in that situation their country attribute should be null. Another example is, when a student does not attempt an exam of a particular subject, his/her obtained marks of that subject should be null. If s/he attends the exam and all the answers are wrong then only s/he scores 0 marks.

Wednesday, December 23, 2009

the role of a high-level data model in the database design process.

After the first step of database design (i.e. requirements collection and analysis), second step is to create a conceptual schema for the database called conceptual database design. The conceptual schema is a concise description of the data requirements of the users and includes detailed descriptions of the data types, relationships and constraints; these are expressed using the concepts provided by the high-level data model. These concepts do not include any implementations details; they are usually easier to understand and can be used to communicate with nontechnical users. The high-level data model enables the database designers to come up with a good conceptual database design.

Wednesday, December 2, 2009

Baglamukhi Mantra

Easiest to just chant as you focus on the Yantra or on she who has two arms, wears yellow garments, with three eyes, a body of effulgence, holding in her hands chisel and hammer, the Bagalamukhi.