数据库练习题库参照.pdfVIP

1. DBMS has many advantages. Then what are two disadvantages of database system? Answer: Two disadvantagesassociatedwith database systemsare listed below. a. Setup of the database system requires more knowledge, money, skills, and time. b. b. The complexity of the database may result in poor performance. 2List ?ve responsibilities of a database management system. Ansers: a. interaction with the ?le manager. b. integrity enforcement. c. security enforcement. d. backup and recovery. e. concurrency control. 3. What are ?ve main functions of a database administrator? Answer: Five main functions of a database administrator are: ? To create the scheme de?nition ? To de?ne the storage structure and access methods ? To modify the scheme and/or physical organization when necessary ? To grant authorization for data access ? To specify integrity constraints 4. Explain the distinctions among the terms primary key, candidate key, and superkey. Answer: A superkey is a set of one or more attributes that, taken collectively, al-lows us to identify uniquely an entity in the entity set. A superkey may contain extraneous attributes. If K is a superkey, then so is any superset of K. Asuperkey for which no proper subset is also a superkey is called a candidate key. It is possible that several distinct sets of attributes could serve as candidate keys. The primary key is one of the candidate keys that is chosen by the databasedesigner as the principal means of identifying entities within an entity set 6. What is logical data independence and why is it important? Answer:Logical data independencemeans that users are shielded from changes in the logical structure of the data, i.e., changes in the choice of relations to be stored. For example, if a relation Students(sid, sname, gpa) is replaced by Studentnames(sid,sname) and Studentgpas(sid,gpa) for some reason, application programs that ope