Essay Question: Explain 4 different means by which constrains are represented in the Conceptual Data Model (CDM).

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1 Question 1 Essay Question: Explain 4 different means by which constrains are represented in the Conceptual Data Model (CDM). By specifying participation conditions By specifying the degree of relationship (1:1, 1:n, or n:m) By specifying entity unique identifiers By specifying additional constraints Question 2 Briefly explain the different options usually available when specifying the system action in response an attempted deletion of a referenced tuple that would result in violating of the referential integrity constraint. Restrict: Disallow the deletion. Cascade: propagate the deletion by deleting all referring tuples. Default: perform the deletion and apply a predefined default value in place of the deleted value in the referring tuples. Question 3 Briefly explain the main drawbacks of the client-multi-server approach of distributed database management The user must manage the connections to remote servers The user must be aware of where the data is located, thus losing the desirable property of location transparency Question 4 Explain three important advantages of SQL routines. SQL routines provide the following advantages: Security: only execute privileges can be given to users, without any other explicit privileges. This enhances security. Efficiency: SQL routines can be stored on the server side, thus resulting in lower network traffic

2 The separation of concerns: developers will spate the internal details of how a certain procedure is implemented from the way it is used. Question 5 Briefly explain what is meant by a lost dependency (also called a hidden dependency), and how this problem can be corrected. A lost dependency is a situation that arises due to the incompatibility of normalization and the preservation of all functional dependencies in the ER diagram. It results in not being able to directly represent some functional dependencies in the ER diagram, thus losing part of the problem specifications. This problem can be corrected by specifying constraints in the additional constraints section to restore the lost dependencies. Question 6 Explain what was the type of interface that was provided in the old file systems approach? The old file systems approach used a type of physical interface whereby the user/application developer had to know the physical locations of the various items in the file in order to retrieve the desired information. All information was retrieved based on their position in the file. Question 7 What was the main disadvantage of this old style of interface? The main disadvantage of such an interface was the high program maintenance overhead in response to changes in the file structure. Changes in the file structures would require re-coding all application programs that referred to affected areas of the original file, resulting in much wasted effort and expense. Question 8 Explain what is meant by logical interface, why it is an improvement over the interface utilized in the old file systems approach? A logical interface is one that does not require the user to refer to data items by location; rather, data items are retrieved based on their logical structure

3 and predefined names. It was an improvement over the interface utilized in the old file systems approach because it avoids having to re-code applications in response to changes in the file structures. Since items are referred to logically, the item references will remain valid in the face of physical file changes. Question 9 How (i.e. in what form) the logical interface is implemented in modern Relational Database Management Systems (RDBMS)? Give an example of a logical request to a relational database. It is implemented in modern relational databases through the relational model and its widely used language SQL. As an example, to retrieve the names and addressed of all students, we might issue the SQL statement: select name, address from student. Question 10 Explain what is meant by unproductive maintenance problem in the old file system approach and how the database approach addresses it. Unproductive maintenance occurs in the old file system approach when applications programs need to be maintained or re-written due to restructuring in the underlying file that is unrelated to the current application and does not lead to any improvements in it. It is usually related to additional requirements in other application programs, resulting in the need to restructure the underlying file(s). This problem is addressed in database management systems by introducing the concept of data independence in which application programs are independent of the file structures. Question 11 Describe briefly the differences between the Information Systems (IS), Information management (IM), and Information Technology (IT) strategies. Information Systems (IS) strategy is focused on the business improvement and what it can be brought about by application of appropriate information systems. The Information Management (IM) strategy focuses on people and the management aspects of the information systems. Finally, the Information Technology (IT) strategy focuses on what technologies should be used, and on the development aspects of the technology.

4 Question 12 Explain the statement: join is a refinement of Cartesian product. Suppose we wish to join tables S(A, B) and T(C, D) on attributes B and C. Write a simple SQL query to make the join using a refinement of Cartesian product. A Cartesian Product operation produces all possible pairs of tuples. We can achieve the effect of join by further restricting the result of the Cartesian product to ensure that the join attributes coming from both tables fulfill the join condition. SELECT * FROM S, T WHERE S.B = T.C; Question 13 The basic operation to normalize a table R regardless of which normal form is to identify an offending functional Dependency (FD) then to successively split it into two tables until all tables are normalized. Describe how this can be done in such a way to guarantee non-loss decomposition? We proceed in each step by projecting R: First, over the left- and right-hand sides of the FD to give S, and then over all the attributes of R except those which form the right-hand side of FD, to give T. This process guarantees non-loss decomposition by the theory of I.J.Heath. Question 14 Explain four different means by which the relational model can represent constraints. 1. By specifying primary keys 2. By specifying foreign keys 3. By specifying alternate keys 4. By specifying general constraints 5. By specifying domains

5 Question 15 In the pure relational model, explain why we can t represent a relationship type by a posted foreign key when both sides of the relationship have optional participation constraints. The pure relational model requires all fields to have a value. No matter which side we use to post a foreign key, we will not be able to guarantee the optional participation since this will require the posted foreign key to have no value. Question 16 Explain the statement: The HAVING clause is to groups what the WHERE clause is to rows. The having clause operates at the level of groups, meaning that each group if filtered individually by applying the condition in the HAVING clause to it. If the group as whole passes the test, it is included in the result, otherwise, it is rejected. The WHERE clause operates at the level of rows, meaning that each row is filtered individually by applying the condition in the WHERE clause to it. If it passes the test, it is included in the result. Question 17 A query written using subqueries can often be re-written using joins and vice versa. Discuss when are subqueries necessary, when are joins necessary, and what factors affect the choice when both options are feasible. A join is necessary when the final table includes data from both tables because fields in tables referred to only inside a subquery are inaccessible in the SELECT Clause. A subquery is necessary when a comparison is to be made with an aggregate function applied to the second table or lists of values from all the rows because this requires two levels of evaluation, not available in queries with joins only. When there is a genuine choice between a join and a subquery, there are two factors to consider: Which formulation seems more naturally based on the English request. Which formulation is likely to be most efficient Question 18 Explain the concept of isolation levels and why this is needed in databases where concurrent access is required? What is the highest isolation level? Isolation levels refer to the ability to define multiple levels of concurrency which differ in how strictly concurrently running transactions are isolated from each other

6 and to also define the level of concurrent access for a transaction. User-selectable isolation levels allow a choice between accuracy of data and speed of execution. The highest level of isolation is known as serializable isolation, which guarantees serializable execution of a transaction. Lower isolation levels may result in one of the concurrency problems occurring, but generally allow quicker execution. Question 19 Discuss the role played by the concept of separation of concerns in modern database management. The separation of concerns facilitates the design of database systems by allowing the designers to focus more on data representations, requirements and processing. The separation of concerns also simplifies the task for programmers developing user processes because it relives them from having to consider all the concerns simultaneously. It also results in facilitating ad hoc querying because the DBMS can translate a logical user query into the more complex physical requirements of the underlying computer system. Finally it leads to logical data independence which in turn results in reduced unproductive maintenance. Question 20 Which data quality issue(s): accuracy, completeness, timeliness, relevance, understandability and trust ability is (are) being compromised in each of the following situations? Explain why? i. The Student Information System does not record the history of the transactions on student marks, including the time and identity of the user who changed your marks in the system. Un-trusted. If an authorized person can maliciously alter students marks in the system without ever being identified, we lose trust in the system. ii. A system posts a notice for a cancelled lecture 5 minutes before the lecture starts Un-timely. Finding out a cancelled lecture only 5 minutes before the lecture is useless to avoid travel to the university. iii. Your request for your course marks returns the marks of all students in a different course. Irrelevant and un-trusted. Irrelevant because it is for a different class, and un-trusted because a system that reveals student marks to unauthorized persons does not convey trust. iv. The registrar wishes to choose a suitable room for a certain class. A request to the system returns a listing of all rooms in the first floor with the designation small or large. Incomplete and not understandable. How about other floors? And what do the terms

7 small or large mean? Question 21 Describe the three key activities of the waterfall model applied to database development. Explain, for each key activity, what document is produced at the end of the activity. 1. Establishing requirements: the users are interviewed and documents are examined to determine the user requirements. The end product is a requirements document that is important as it acts as a contract between the user and the developer. 2. Data analysis: is the process of converting the requirements document into a conceptual data model that is independent of the implementation environment. Its end product is the conceptual data model and it is important because its acts as a formal system specification throughout the development process to follow. 3. Database design: is the process of developing a system-specific design for the intended database. Its end product is the logical schema and it is important because it forms the basis of the subsequent system implementation. Question 22 Explain what is meant by weak entity types in a conceptual data model? A weak entity type is an entity type whose entities cannot exist without the existence of another identifying entity type. Question 23 Explain one key advantage of using SQL routines in database development. 1. Centralizing the location of functionality: Routines are held in a central place in the database and are made accessible to all applications. In case the definition of a routine needs to be changed, it will need to be updated only once, not in many places. 2. Improving security: Instead of granting access rights to all users, we can only grant execute right to the stored routines, which will help control the types of access allowed for the users. 3. Improving Efficiency: Routines that are defined within the database on the server side do not need to use communication networks between the client and the server, and therefore improving efficiency. Question 24 Explain what is meant by Data Type Definition (DTD) in the context of the extended Markup Language (XML).

8 A DTD is a way to define the structure or type of an XML document and to define which documents represent valid instances of this definition. Question 25 Explain what is meant by transaction management and why it is important in a multi-user E-nvironment? Transaction management aims to eliminate the interference between multiple operatio on the same data while allowing a maximum amount of availability. In especially important in multi-user environments because it allows multiple users to access and update the database efficiently without risk if interference among their transactions. Question 26 Explain why a compiled program including embedded SQL statements is not portable across different DBMSs from different Vendors? It would not be portable because the compiled SQL code would be specific to the DBMS for which it was compiled. The Open Data Base Connectivity (ODBC) approach has been developed to overcome this limitation. Question 27 Explain why a composite query involving a correlated nested sub-query can be inefficient. A composite query involving a correlated nested sub query can sometimes be inefficient because the subquery will need to be evaluated many times, once for each tuple considered in the outer query. For example, returning the employees whose salary is higher than the average salary of all employees in his/her department will require the average salary for each department to be evaluated once for each employee in it. Question 28 Explain when you would need to add a general constraint to a relational database definition in the theoretical database model. A general constraint would be needed if the constraint cannot be represented in the E-R model or if a relationship with a mandatory participation condition is implemented using a posted foreign key from the mandatory side to the other side.

9 Question 29 Explain when it would be necessary to use a subquery as opposed to using a join? A subquery is necessary when a comparison is to be made with an aggregate function applied to the second table or lists of values from all the rows. Question 30 Briefly explain the two possible outcomes of a transaction in a transaction management system, and why are transactions needed in a shared environment? The two possible outcomes are: 1. The transaction succeeds and all its statements are carried out, or 2. One of its statements fails resulting in all the changes made getting rolled back. Transactions are needed in a shared environment to prevent different processes from interfering with each other in such a way that incorrect results are produced. Question 31 Briefly explain the main benefits of producing a normalized database. 1. Reducing unnecessary redundancy 2. The elimination of insertion, deletion and update anomalies Question 32 Briefly explain the main phases of developing a database system using the waterfall model. 1. establishing requirements, produces the data requirement specifications 2. data analysis, produces the conceptual model 3. database design, resulting in the logical schema 4. implementation, resulting in an initial schema and database 5. testing against the requirements specifications, resulting in the released schema and database 6. maintenance, a continuous activity Question 33 Explain the main advantages of using procedures in database systems. 1. Improving security by giving only execution rights to procedures on critical data. 2. Improving performance since procedures can be stored on the server, eliminating the

10 need to transfer large data sets to/from the server. 3. Achieving a better measure for the separation of concerns. Explain the difference between cascade, default and restrict options when deleting a references tuple that would result in violating referential integrity. Question 34 Explain how the global and distribution schemas are used in a distributed database system? A global schema is like a logical schema for the entire database while a distribution schema is like a storage schema which defines where data is located. They are used to provide a single unified logical schema for the entire database and to help achieve location transparency, where the user is not required to be aware of where the data is located. Question 35 What is the different between a table and a relation? Mention the relatedrules Arelation is an abstract concept consisting of a set of tuples of attribute values in any Order. Provided that it obeys the following rules, a table might be aconcrete depiction of a relation Rule 1: atomic entries Rule 2: consistent column values Rule 3: unique column names Rule 4: distinct rows Rule 5: insignificance of order

11 AMSWER :

A subquery is a nested query inserted inside a large query Generally occurs with select, from, where Also known as inner query or inner select,

Sub queries A subquery is a nested query inserted inside a large query Generally occurs with select, from, where Also known as inner query or inner select, Result of the inner query is passed to the main