Skip to main content

Explain the characteristics that make a relation different from a file or table?

 Ordering  of tuples in a relation 

r Relation defined as a set of tuples of elements have no order among them.


Ordering of values within a tuple and an alternative definition of relation

Order of attributes and values is not that important.

As long as correspondence between attributes and values maintained.


Alternative definition of relation

Tuples considered as a set of (<attributes>, <value>) pairs.

Each pairs gives the value of the mapping from an attribute A1 to a value v1 from dom(A1).


Use the first definition of relation

Attributes and the values within tuples are ordered.


Values and Nulls in tuple

  1. Represent the values of attributes that may be unknown or may not apply to a tuple.
  2. Meanings for null values
  • Value Unknown
  • Value exists but is not available
  • Attribute does not apply to this tuple (also known as value undefined)
  1. Each value in a tuple is atomic.
  2. Flat relational model
  • Composite and multi-valued attributes not allowed.
  • First Normal Form assumption.
  1. Multi-valued attributes
  • Must be represented by separate relations .
    Example: Emp_phone
  1. Composite attributes
  • Represented only by simple component attributes in basics relational model.
    Example- Emp-name



Labels:

Comments

Post a Comment

Popular posts from this blog

Define the following terms: Entity, Attribute, Attribute Value, Relationship Instance, Composite Attribute, Multi-valued Attribute, Derived Attribute, Complex Attribute, Key Attribute, Value set(Domain), Recursive Relationships?

Entity – An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity. Attribute – In general, an attribute is a characteristic. In a database management system (DBMS), an attribute refers to a database component, such as a table. It also may refer to a database field. Attributes describe the instances in the row of a database. 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
Post a Comment
Read more

Define Primary Key, Candidate Key and Super Key?

Super Key Super Key is defined as a set of attributes within a table that uniquely identifies each record within a table. Super Key is a super-set of Candidate key. Example - Suppose we have a table that holds all the managers in a company, and that table is called Managers. The table has columns called ManagerID, Name, Title, and DepartmentID. Every manager has his/her own ManagerID, so that value is always unique in each and every row. This means that if we combine the ManagerID column value for any given row with any other column value, then we will have a unique set of values. So, for the combinations of (ManagerID, Name), (ManagerID, TItle), (ManagerID, DepartmentID), (ManagerID, Name, DepartmentID), etc – there will be no two rows in the table that share the exact same combination of values, because the ManagerID will always be unique and different for each row. This means that pairing the Manager ID with any other column(s) will ensure that the
1 comment
Read more

Explain the four actors on the scene of Database?

For a small personal database, one person defines, constructs and manipulates the database . For a large organization, many people are involved in the design, use and maintenance of the large database Two types of actors of DBMS: 1. Actors on the Scene (day-to-day use of large database ) 2. Workers behind the Scene ( work to maintain database environment and are not involved in its contents ) Actors on the Scene Database administrators (DBA) are responsible for:  Authorizing access to the database  Coordinating and monitoring its use  Acquiring software and hardware resources Database designers are responsible for:  Identifying the data to be stored  Choosing appropriate structures to represent and store this data . End users - People whose jobs require access to the database.  Types of End Users • Casual end users • Naive or p
Post a Comment
Read more