期末试题标准答案-数据库技术与应用.doc

标准答案格式 北京邮电大学2007——2008学年第一学期 《数据库技术与应用》期末考试试题标准答案 True-False Questions（20） 题号 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 答案 T T F T T T F T T F T F F F T F T T T T Multiple Choice Questions（30） 题号 1 2 3 4 5 6 7 8 9 10 答案 E B C D E A D D E E 题号 11 12 13 14 15 16 17 18 19 20 答案 B A C C A B B C A C 题号 21 22 23 24 25 26 27 28 29 30 答案 A E D B B D E A B D Fill in the Blank Questions（10） 题号 答案 1 Structured Query Language 2 DISTINCT 3 1 4 TOP 5 primary key 6 UPDATE 7 ALTER 8 transaction 9 exclusive 10 static Essay Questions（30） Briefly describe subqueries and joins. Explain when each is not an acceptable alternative for the other. Subqueries and joins are both methods for retrieving data from multiple tables. Subqueries involve nesting one SELECT statement within another. The nested SELECT is used as part of a condition in the WHERE clause of the first SELECT statement. The nested SELECT statement can return a set of records from one table, which are then used in a logical operator within the parent SELECT query. A join combines records from each table into concatenated records containing the fields of both tables. The records are concatenated based on matching values in similar columns in the two tables. Subqueries cannot be used in situations where the results to be displayed include attributes from more than one table. Joins cannot be used as an alternative to a correlated subquery. Briefly describe the process of converting an extended E-R model into a relational database design. To convert an extended E-R model into a relational database design, first tables and columns must be defined for each entity attribute. The attributes of the entity are represented as the columns of the table. Primary keys are then selected for each table, after which the relationships between the entities are represented through the creation of foreign keys. Next, constraints are specified. Finally the normalization is verified. The