数据库系统基础教程答案ch4

b)

A captain cannot exist without a team. However a player can (free agent). A recently formed (or defunct) team can exist without players or colors.

c)

Children can exist without mother and father (unknown).

4.3.2 a)

The keys of both E1 and E2 are required for uniquely identifying tuples in R b)

The key of E1 c)

The key of E2 d)

The key of either E1 or E2

4.3.3

Special Case: All entity sets have arrows going into them i.e. all relationships are 1-to-1

Any Ki

Otherwise: Combination of all Ki's where there does not exist an arrow going from R to Ei.

4.4.1

No, grade is not part of the key for enrollments. The keys of Students and Courses become keys of the weak entity set Enrollments.

4.4.2

It is possible to make assignment number a weak key of Enrollments but this is not good design (redundancy since multiple assignments correspond to a course). A new entity set Assignment is created and it is also a weak entity set. Hence the key attributes of Assignment will come from the strong entity sets to which Enrollments is connected i.e. studentID, dept, and CourseNo.

4.4.3 a)

b)