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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



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