Natural Join
A further special case of the join operation R ./ S is an equijoin in which equalities are speci ed on all elds having the same name in R and S. In this case, we can simply omit the join condition; the default is that the join condition is a collection of equalities on all common elds. We call this special case a natural join, and it has the nice property that the result is guaranteed not to have two elds with the same name. The equijoin expression S1 ./R:sid=S:sid R1 is actually a natural join and can simply be denoted as S1 ./ R1, since the only common eld is sid. If the two relations have no attributes in common, S1 ./ R1 is simply the cross-product.
Calling Built-In Applications Using Intents Until this point, you have seen how to call activities within your own application. One of th...