Dbms Joins

Question

Joins and Cross products are associative or commutative?

Answer 1

Cross Product      : is both associative and commutative

Natural Inner Join : is also both associative and commutative.

Note:

1) In a relation order of the tuples and columns is insignificant.

Comments

Natural inner join is same as natural join?

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Every join operator has two components namely

1) Join Condition , which dictates:

          i) how to handle attributes in the resultant relation

          ii) how to compare tuples.

          There are two join conditions

              a) Natural

                      i) take union of attributes

                      ii) match on common column

             b) On

                      i) Duplicates of attributes are also allowed

                      ii) Join condition is explicitly specified. eg(on a.salary> b.salary)

 2) Join Type, which dictates

                  i) how to handle unmatched tuples. eg; as you may know Full outer,left outer and right outer.

So the term Natural join alone is incomplete, but is predominantly used to specify Natural Inner Join. And obviosuly Left and right outer join types are not commutative.