Relational Model #2

Relational Query Languages

• Selection of tuples



Relation r



after selection




• If I want to select tuples with A=B and D>5 in relation r,
• σA=B and D>5 (r)

• Selection of Attributes

• Projection
• Π A, C (r)

after projection

• Cartesian Product - Cross product!

relations r and s

•  Cartesian product returns every possible pair in the result.

r ╳ s

• Union of two relations

relations r and s

• r ∪ s =

Union of r and s

• Set difference between two relations

relations r and s

• r - s = 

differences

• Set intersection between two relations

relations r and s

• r ∩ s =

intersection

• Natural Join
• Let r and s be relations on schemas R and S respectively. Then, the 'natural join' of relations R and S is a relation on schema R∪ S obtain as follows:
1. Consider each pair of tuples tr from r and ts form s.
2. If tr and ts  have the same value on each of the attributes in R∩ S, add a tuple t to the result, where
• t has the same value as tr on r
• t has the same value as ts on s

relations r and s

•  r ⋈ s

Natural join r and s