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Figure : Intersection DIFFERENCE Example Figure : DIFFERENCE
CARTESIAN PRODUCT
The Cartesian Product is also an operator which works on two sets. It
is sometimes
called the CROSS PRODUCT or CROSS JOIN.
It combines the tuples of one relation with all the tuples of the other
relation. CARTESIAN PRODUCT example Figure : CARTESIAN
PRODUCT JOIN Operator
JOIN is used to combine related tuples from two relations:
In its simplest form the JOIN operator is just the cross product of the
two
relations.
As the join becomes more complex, tuples are removed within the
cross product
to make the result of the join more meaningful.
JOIN allows you to evaluate a join condition between the attributes of
the
relations on which the join is undertaken.
The notation used is
R JOINjoin condition S JOIN Example Figure : JOIN Natural Join
Invariably the JOIN involves an equality test, and thus is often
described as an equi-join.
Such joins result in two attributes in the resulting relation having
exactly the same value.
A `natural join' will remove the duplicate attribute(s).
In most systems a natural join will require that the attributes have the
same name
to identify the attribute(s) to be used in the join. This may require a
renaming
mechanism.
If you do use natural joins make sure that the relations do not have
two attributes
with the same name by accident. OUTER JOINs
Notice that much of the data is lost when applying a join to two
relations. In some cases
this lost data might hold useful information. An outer join retains the
information that