Equijoin
A common special case of the join operation R ./ S is when the join condition consists solely of equalities (connected by ^) of the form R:name1 = S:name2, that is, equalities between two elds in R and S. In this case, obviously, there is some redundancy in retaining both attributes in the result. For join conditions that contain only such equalities, the join operation is re ned by doing an additional projection in whichS:name2 is dropped. The join operation with this re nement is called equijoin. The schema of the result of an equijoin contains the elds of R (with the same names and domains as in R) followed by the elds of S that do not appear in the join conditions. If this set of elds in the result relation includes two elds that inherit the
same name from R and S, they are unnamed in the result relation. We illustrate S1 ./R:sid=S:sid R1 in Figure 4.13. Notice that only one eld called sid appears in the result.
