Show that step 2 of the method used in Section 16.3.1 to perform a join using a semi- join does in fact generate the semi-join. For simplicity, assume that the join we are attempting is a natural join. That is, prove that ?attributes(T1) (T1 T2) = ?attributes(T1)(T1 ?attributes(join-condition)(T2))
What will be an ideal response?
This a special case of the rule for pushing projections through joins in Section 11.2.
For every tuple t ? T1 T2 there is a tuple t
? T1 ?attributes(join-condition)(T2)
that agrees with t on all attributes of T1 and vice versa. Thus the above two expressions
are equivalent.
You might also like to view...
?A(n) ____ reference identifies a cell by its location in relation to other cells and changes when a formula is moved or copied.
A. ?absolute B. ?relative C. ?template D. ?vector
A technician is configuring a new SOHO multifunction wireless router at a customer’s location to provide network access to several wireless devices. The technician is required to protect the customer’s private network from unauthorized access while performing the initial router setup as well as during normal operation after the configuration is completed. Which of the following is the best step for the technician to take in securing the router to meet this requirement?
A. Enable DHCP on the router and configure IP address reservations for the wireless clients. B. Change the router’s default administrative password. C. Disable the router’s radio in the wireless network settings. D. Configure MAC filtering on the router and allow access to only the technician’s computer.