For each table, identify each of the following when possible:

a$.$ The primary key

b$.$ A superkey

c$.$ A candidate key

d$.$ The foreign key$($s$)$

e$.$ A secondary key

AviaCo is an aircraft charter company that supplies on$-$demand charter flight services using a fleet of four aircraft. Aircraft are identified by a unique registration number. Therefore, the aircraft registration number is an appropriate primary key for the AIRCRAFT table.

The nulls in the CHARTER table's CHAR$\_$COPILOT column indicate that a copilot is not required for some charter trips or for some aircraft. Federal Aviation Administration $($FAA$)$ rules require a copilot on jet aircraft and on aircraft that have a gross take$-$off weight over $12,500$ pounds. None of the aircraft in the AIRCRAFT table are governed by this requirement; however, some customers may require the presence of a copilot for insurance reasons. All charter trips are recorded in the CHARTER table.

Note: Earlier in the chapter, you were instructed to avoid homonyms and synonyms. In this problem, both the pilot and the copilot are listed in the PIlOT table, but EMP$\_$NUM cannot be used for both in the CHARTER table. Therefore, the synonyms CHAR$\_$PIlOT and CHAR$\_$COPIlOT were used in the CHARTER table.

Although the solution works in this case, it is very restrictive, and it generates nulls when a copilot is not required. worse, such nulls proliferate as crew requirements change. For example, if the AviaCo charter company grows and starts using larger aircraft, crew requirements may increase to include flight engineers and load masters. The CHARTER table would then have to be modified to include the additional crew assignments; such attributes as CHAR$\_$FlT$\_$ENGINEER and CHAR$\_$lOADMASTER would have to be added to the CHARTER table. Given this change, each time a smaller aircraft flew a charter trip without the number of crew members required in larger aircraft, the missing crew members would yield additional nulls in the CHARTER table.

You will have a chance to correct those design shortcomings in Problem $27.$ The problem illustrates two important points:

$1.$ Don't use synonyms. If your design requires the use of synonyms, revise the design!

$2.$ To the greatest possible extent, design the database to accommodate growth without requiring structural changes in the database tables. Plan ahead and try to anticipate the effects of change on the database.