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Logical Model Operations

  • In relation databases, attribute values must be atomic
  • Select
    • o_p(r)
    • p is the selection predicate
      • Can use =, =/, >, >=, <, <=
    • Ex: Select the tuples of the relation where the instructor is in the physics department
      • o(deptname)="Physics"^(instructor)
  • Unary operator (project)
    • Symbol is like a Vertical rectangle w/ no bottom
    • Parameters are attributes
    • Gets only the attributes asked for
  • Cartesian product (x) combines info from two relations
    • Constructs a tuple out of result w/ every possible combo
  • Composition
  • Join
    • Cartesian product between two tables w/ foreign key in both
    • Select to make sure foreign key = primary key
  • Union
    • Can join two selects
      • Both must have same number of attributes
      • Must have compatible domains
  • Set difference (minus symbol)
  • Rename:
    • p_x
  • Many queries can be written in different ways
  • Filter first, join later
    • Reduces compute power/time complexity