Condition Joins
The most general version of the join operation accepts a join condition c and a pair of relation instances as arguments, and returns a relation instance. The join condition is identical to a selection condition in form. The operation is de ned as follows:
R ./c S = c(R S)
Thus ./ is de ned to be a cross-product followed by a selection. Note that the condition c can (and typically does) refer to attributes of both R and S. The reference to an attribute of a relation, say R, can be by position (of the form R:i) or by name (of the form R:name).
As an example, the result of S1 ./S1:sid
appears in both S1 and R1, the corresponding elds in the result of the cross-product
S1 R1 (and therefore in the result of S1 ./S1:sid
are inherited from the corresponding elds of S1 and R1.
