Page 221 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE
P. 221
Database Management Systems/Managing Database
Notes
Figure 12.6
The selection condition may be moved to the join operation. The selection condition given in
the Figure 12.6 is: sname = DBMS and grade = A. Both of these conditions belong to different
tables, as s name is available only in the SUBJECTS table and grade in the MARKS table. Thus,
the conditions of selection will be mapped accordingly as shown in the Figure 12.7. Thus, the
equivalent expression will be:
Figure 12.7
The expected size of SUBJECTS and MARKS after selection will be small so it may be a good idea
to first join MARKS with SUBJECTS. Hence, the associative law of JOIN may be applied.
214 LOVELY PROFESSIONAL UNIVERSITY
