Additional operations are defined in terms of the fundamental operations. They do not add power to the algebra, but are useful to simplify common queries. The Set Intersection Operation Set intersection is denoted by , and returns a relation that contains tuples that are in both of its argument relations. Often we want to simplify queries on a cartesian product. This type of operation is very common, so we have the natural join , denoted by a sign.
Natural join combines a cartesian product and a selection into one operation.
It performs a selection forcing equality on those attributes that appear in both relation schemes. Duplicates are removed as in all relation operations. Joining borrow and customer relations. We can now make a more formal definition of natural join. Consider and to be sets of attributes.