Student

Only available on StudyMode
  • Download(s) : 29
  • Published : December 17, 2012
Open Document
Text Preview
DATABASE FUNCTIONAL DEPENDENCY
A functional dependency occurs when one attribute in a relation uniquely determines another attribute. This can be written A -> B which would be the same as stating "B is functionally dependent upon A." EXAMPLE

In table listing employee characteristics including Social Security Number (SSN) and name, it can be said that name is functionally dependent upon SSN (or SSN -> name) because an employee's name can be uniquely determined from their SSN. However, the reverse statement (name -> SSN) is not true because more than one employee can have the same name but different SSNs. QUESTION 2

ENTITY RELATIONAL MODEL(ERM)
It is an abstract way to describe a database. It usually starts with a relational database, which stores data in tables. Some of the data in these tables point to data in other tables. For instance your entry in the database could point to several entries for each of the phone numbers that are yours. The ER model would say that you are an entity, and each phone number is an entity, and the relationship between you and the phone numbers is 'has a phone number'. Diagrams created to design these entities and relationships are called entity–relationship diagrams or ER diagrams. CONCPETUAL SCHEMA

Conceptual Schema is sometimes called logical schema. It describes the stored data in terms of the data model of the DBMS. In a relational DBMS, the conceptual schema describes all relations that are stored in the database

Alternate key - An alternate key is any candidate key which is not selected to be the primary key  Candidate key - A candidate key is a field or combination of fields that can act as a primary key field for that table to uniquely identify each record in that table.  Primary key - a primary key is a value that can be used to identify a unique row in a table Foreign key - a foreign key (FK) is a field or group of fields in a database record that points to a key field or group of fields forming a key...
tracking img