BIRKBECK (University of London)

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1 BIRKBECK (University of London) BSc Examination for Internal Students School of Computer Science and Information Systems Database Management COIY028U - Course Unit Value: 1/2 May 2006 : Afternoon This paper contains four questions. You should attempt exactly three questions. Page 1 of 5

2 Page 2 of 5 1. Answer the following three parts. a. Suppose that you have a database schema consisting of two relation schemas EMP and DEPT such that schema(emp) = {EN, EA, DN} and schema(dept) = {DN, DA, MN}. (EN stands for Employee Name, EA stands for Employee Address, DN stands for Department Name, DA stands for Department Address, and MN stands for Manager Name.) The primary key of EMP is EN and the primary key of DEPT is DN. Construct and Entity Relationship Diagram (ERD) from the database schema {EMP, DEPT}, explaining in generic terms how you have reverse-engineered the ERD from the database schema. b. Let r be a relation over EMP and s be a relation over DEPT. Discuss different policies of maintaining referential integrity given that the tuple <John, London, Maths> is inserted into r and the tuple <Computing, London, Jane> is deleted from s. c. Show how ISA relationships can be used to model inheritance between entity types. How can the Universal Relation Schema Assumption (URSA) help disambiguate naming conflicts due to inheritance from multiple entity types. Use an example based on an extension of your ERD from part (a) to illustrate your answer. c Birkbeck College 2006 COIY028U CONTINUED

3 Page 3 of 5 2. Answer the following three parts. a. Briefly comment on the support for null values in SQL. Explain the equality rule for null values in SQL with reference to an example database of your choice. b. Let r be a relation over a relation schema R and X be a subset of the set of attributes of R. Suppose that s is a view defined as the projection of r onto the set of attributes X of R. 1. Give the SQL query for creating the view s. 2. A user interacting with the view s may wish to update a row in s without explicit reference to the effect this update will have on r. What condition would you impose on the set of attributes X, used to create the view, in order that an insertion or deletion of a single row in s could be effected by simply inserting or deleting a single row from r. c. Write a sentence or two explaining each of the following: 1. Three levels of data abstraction. 2. Unique Name Assumption. 3. growth independence. 4. ID relationship type. 5. Complex objects. c Birkbeck College 2006 COIY028U TURN OVER

4 Page 4 of 5 3. Answer the following three parts. a. Why do multi-valued attributes violate first normal form (1NF)? Assume an entity type, PERSON having multi-valued attribute PHONES, denoting the set of phone numbers that the person may have. Show how you could express the same information in a 1NF relational database. b. Briefly indicate the need for object-relational databases and discuss three features that a relational database system may support in order for it to qualify as objectrelational. c. Let R be a relation schema, with schema(r) = {A,B,C,D,E,F,G}, together with a set of functional dependencies, F = {A BC, BD E, EC A, FG E}. Compute the closure of X with respect to F for all sets X containing one and two attributes of schema(r). c Birkbeck College 2006 COIY028U CONTINUED

5 Page 5 of 5 4. Answer the following three parts. a. Define Third Normal Form (3NF) using an example. Give an example of a relation schema, which is in 3NF but not in BCNF (Boyce- Codd Normal Form). Explain why a database designer may comprise for relation schemas which are in 3NF but not in BCNF. b. Define the notion of a redundancy problem using a simple example. Argue with the aid of a simple example that a relation schema is in BCNF if and only if it has no redundancy problems. c. Give an efficient algorithm for finding a single key in a relation schema R with respect to a set of functional dependencies F over R. Illustrate your algorithm using a simple example. c Birkbeck College 2006 COIY028U END OF PAPER