Dbms Interview Question Answers.
1. What are data and information, and how are they related in a database?
Answer: Data is recorded facts and figures, and information is knowledge derived from data. A database stores data in such a way that information can be created.
2. What is a data mart?
Answer: A data mart is a collection of data smaller in scope and size than a data warehouse. It is dedicated to data from a particular business component of business functional area. A data mart may function as a subset of a larger data warehouse. Users of a data mart are usually knowledgeable analysts in the business area using the data mart.
3. What are insertion and deletion anomalies?
Answer: A deletion anomaly occurs when, by deleting the facts about one entity, we inadvertently delete facts about another entity; with one deletion, we lose facts about two entities. For example, if we delete the tuple for Student 001289 from a table, we may lose not only the fact that Student 001289 is in Pierce Hall, but also the fact that he has $200 left in his security deposit. An insertion anomaly happens when we encounter the restriction that we cannot insert a fact about one entity until we have an additional fact about another entity. For example, we want to store the fact that the security deposit for Pierce Hall is $300, but we cannot enter this data into the Student relation until a student registers for Pierce Hall.
4. What is a dependency graph?
Answer: A dependency graph is a diagram that is used to portray the connections between database elements. For example, a change in a table can potentially impact relationships, views, indexes, triggers, stored procedures, and application programs. These impacts need to be known and accounted for before making database changes.
5. What is an SQL subquery?
Answer: An SQL subquery is a means of querying two or more tables at the same time. The subquery itself is an SQL SELECT statement contained within the WHERE clause of another SQL SELECT statement, and separated by being enclosed in parenthesis. Some subqueries have equivalent SQL join structures, but correlated subqueries cannot be duplicated by a join..
6. What are stored procedures, and how do they differ from triggers?
Answer: A stored procedure is a program that is stored within the database and is compiled when used. They can receive input parameters and they can return results. Unlike triggers, their scope is database-wide; they can be used by any process that has permission to use the database stored procedure.
7. Explain what we mean by an ACID transaction.
Answer: An ACID transaction is one that is atomic, consistent, isolated, and durable. Durable means that database changes are permanent. Consistency can mean either statement level or transaction level consistency. With transaction level consistency, a transaction may not see its own changes. There are four transaction isolation levels: read committed, read uncommitted, repeatable read and serialization. Atomic means it is performed as a unit.
8. What does it mean when we say that a relation is in Boyce-Codd Normal Form (BCNF)?
Answer: A relation is in BCNF when every determinant in the relation is a candidate key. This means that any possible primary key can determine all other attributes in the relation. Attributes may not be determined by non-candidate key attributes or part of a composite candidate key. Thus it is said "I swear to construct my tables so that all nonkey columns are dependent on the key, the whole key and nothing but the key, so help me Codd!"
9. Define a surrogate key, describe the ideal primary key and explain how surrogate keys meet this ideal.
Answer: The ideal primary key is short, numeric and fixed. A surrogate key is a unique, DBMS-supplied identifier intended to be used as the primary key of a table. Further, the DBMS will not allow the value of a surrogate key to be changed. The values of a surrogate key have no meaning to the users and are usually hidden on forms and reports. By design, they are short, numeric and fixed and thus meet the definition of the ideal primary key
10. Briefly describe the four JDBC driver types that Sun defines.
Answer: Type 1 drivers provide a bridge between Java and ODBC. Types 2-4 drivers are written entirely in Java, but differ as to how they connect to the DBMS. Type 2 drivers rely on the DBMS product for intermachine communication, if any. Type 3 drivers translate JDBC calls into a DBMS-independent network protocol. Type 4 drivers translate JDBC calls into a DBMS-dependent network protocol.
11. What are sequences, and what are the possible problems when using them to create surrogate keys?
Answer: A sequence is an object that generates a sequential series of unique numbers. Sequences are most often used to provide values for surrogate keys. However, there are three problems with using sequences. First, a developer can use a defined sequence for any purpose; and if a sequence is used for purposes other than the surrogate key, some values will be missing. A second problem is that there is nothing in the schema that prevents someone from issuing an INSERT statement that adds data to the table but that does not use the sequence. Finally, it is possible to use the wrong sequence when putting data into a table.
12. Name and describe three types of binary relationships.
Answer: 1:1 - a single entity instance of one type is related to a single-entity instance of another type. 1:N - a single entity instance of one type is related to many-entity instances of another type. M:N - many-entity instances of one type relate to many-entity instances of another type.
13. Explain the difference between two and three-tier architectures.
Answer: A three-tier architecture includes a client and two server layers. The application code is stored on the application server and the database is stored on the database server. A two-tier architecture includes a client and one server layer. The database is stored on the database server.
14. Describe domain constraints.
Answer: Domain constraints include entity integrity and referential integrity. The domain is a set of values that may be assigned to an attribute. The entity integrity rule states that no part of a primary key cannot be null. Referential integrity states that each foreign key value must match a primary key value or be null.
15. Compare a thin and a fat client.
Answer: A fat client is a PC that is responsible where most processing occurs on the client rather than the server. Some of the fat client activities include processing, presentation logic, and business rules logic. A thin client is a PC where only limited processing occurs. In these cases, more processing should occur on the application server.
16. What is the archtetype/instance pattern?
Answer: The archetype/instance pattern occurs when one entity tracks occurrences of another entity. A common example is the relationship between CLASS and SECTION, where the actual occurrence of a class happens when students register for a SECTION of that CLASS. The archetype/instance pattern is implemented as an ID-dependent relationship.
17. What is the difference between horizontal and vertical partitioning?
Answer: Horizontal partitioning is where some rows of a table are placed into the base relations at one site and other rows are placed at another site. Vertical partitioning is where some columns of a table are placed into the base relations at one site and other columns are placed at another site but each all of these relations must share a common domain.
18. Describe the naming of Relationships?
Answer: Relationships are verb phrases and represent actions usually in the present tense. The name should exclude vague names such as "has". The name should explain what action is being taken and possibly why it is important.
19. Describe middleware applications that ease the connection of databases to Web applications.
Answer: Both ASP and ColdFusion are middleware that ease the connection of databases to Web applications. Both require the use of several files. Both use tags and are executed on the server. Both Internet Explorer and Netscape can access these files. The programmer does not need to be concerned with the client when they work with this middleware.
20. What is a DBMS?
Answer: DBMS stands for Database Management System. A DBMS receives requests from applications and translates those requests into actions on a specific database. A DBMS processes SQL statements or uses other functionality to create, process and administer databases.
21. Who can make up a systems or database team? Provide a brief explanation of each.
Answer: The team includes a system analyst who identifies the need for information services to meet opportunities of the business, database analysts who design the database, users who monitor that the system will meet their needs, programmers who write computer programs, database and data administrators who have responsibility for current and future databases and other technical experts.
22. Explain minimum and maximum cardinality.
Answer: Minimum cardinality is the minimum number of instances of an entity that can be associated with each instance of another entity. Maximum cardinality is the maximum number of instances of an entity that can be associated with each instance of another entity.
23. Describe ODBC
Answer: ODBC is a standard that contains an interface that provides a common language for application programs to access and process SQL databases. In order to use ODBC, a driver, server name, database name, user id, and password are required. ODBC is important for Internet applications and has gained wide acceptance.
24. Explain why an information gap exists in most organizations.
Answer: One reason that an information gap exists is the fact that systems have been developed in separate, segmented efforts. This has helped the data from being stored in an integrated database and thus the data is in an inconsistent structure. The other reason for the gap is that most systems are created to support the operational aspect of an organization. The systems were not developed for decision making.
25. What is denormalization and why would someone consider doing so?
Answer: Denormalization is the process of taking normalized relations and changing them so that they are not longer normalized. This process may lead to anomalies and create data redundancy as negative consequences. However, the revised relations should improve database performance.
26. Describe why operational and informational systems are separate.
Answer: Operational systems are used to run the business in real time on a daily basis and contain current data. Non-managers and non-analysts work in this type of system. Operational systems must process large volumes of data. Informational systems are used to support decisions and contain historical data. Managers and analysts work in this type of system. Informational systems have periodic batch updates.
27. What are some disadvantages of a standard language such as SQL?
Answer: A standard language can hinder the effort to create a new language. One standard is never enough to meet all of the business needs. A standard can be a compromise among interested parties which can cause the standard to not be ideal. If a standard is altered by some, than portability between platforms could be hurt.
28. Why is XML a better markup language than HTML?
Answer: XML is a better markup language than HTML, primarily because XML provides a clear separation between document structure, content, and materialization. Symbols cannot be used ambiguously with XML.
29. Explain concurrency transparency.
Answer: Concurrency transparency is where each transaction in a distributed database is treated as if it is the only one in the system. Therefore if several transactions are running at one time, the results will be the same as if each transaction was run in serial order. The transaction manager helps to provide concurrency control. The three methods that may be used are locking, versioning, and timestamping.
30. Write an SQL SELECT statement to display all the columns of the STUDENT table but only those rows where the Grade column is greater than or equal to .
Answer: SELECT * FROM STUDENT WHERE Grade >= 90;
31. What is the difference between simple elements and complexType elements?
Answer: Simple elements have only one data value. ComplexType elements can have multiple elements nested within them. ComplexTypes may also have attributes. The elements contained in a complexType may be simple or other complexTypes. ComplexTypes may also define element sequences.
32. Explain the difference between attributes and identifiers.
Answer: Entities have attributes. Attributes are properties that describe the entity's characteristics. Entity instances have identifiers. Identifiers are attributes that name, or identify, entity instances.
33. The range of database applications can be divided into five categories. Explain the five different categories.
Answer: Databases can support from a single user (personal database) up to supporting the requests of the world (internet database). In between, a database can support a workgroup (a relatively small group of people), department database (a functional unit in an organization such as marketing), or an enterprise database (entire organization).
34. You have two tables, EMPLOYEE and COMPUTER that are in a one-to-one relationship. The foreign key is EmpNumber in COMPUTER which references EmpNumber as the primary key of EMPLOYEE. Explain what must be done to convert the one-to-one EMPLOYEE-COMPUTER relationship to a one-to-many relationship where one employee can have more than one computer.
Answer: In the one-to-one relationship, there will be a constraint on EmpNumber as a foreign key in COMPUTER stating that EmpNumber must be unique. To convert the relationship to a one-to-many relationship, just drop this constraint.
35. Describe the difference between data and database administration.
Answer: Data administration is responsible for the overall management of data resources. Some of the core roles include the creation of data polices, procedures and standards, resolve data ownership issues, and manage the information repository. Database administration is physical database oversight. Some of the core duties include the selection of the DBMS and software tools, the installation and upgrade of the DBMS, and database performance tuning.
36. In OLE DB, what is the difference between an interface and an implementation?
Answer: An OLE DB interface is specified by a set of objects, and the properties and methods that they expose, and OLE DB defines standardized interfaces. An object need not expose all of its properties and methods in a given interface. An OLE DB implementation defines how the object supports the interface. The implementation is completely hidden from the user. Thus developers of an object are free to change the implementation whenever they want, but they should not change the interface without consulting their users.
37. What triggers does SQL Server support?
Answer: SQL Server 2000 supports INSTEAD OF and AFTER triggers only. There is no SQL Server support for BEFORE triggers. A table may have one or more AFTER triggers for insert, update and delete actions; AFTER triggers may not be assigned to views. A view or table may have at most one INSTEAD OF trigger for each triggering action of insert, update or delete.
38. What are the major functions of the database administrator?
Answer: Managing database structure, controlling concurrent processing, managing processing rights and responsibilities, developing database security, providing for database recovery, managing the DBMS and maintaining the data repository.
39. Explain locking.
Answer: Locking is denying others the ability to update a record until someone completes the update or releases the record. Locking can occur on many different levels in a database. It can occur at the database, table, record, or field level. A lock can be shared (another can read the record while an update is in progress) or exclusive (no one can read the record while an update is in progress).
40. What are the three types of data sources used with ODBC?
Answer: An ODBC file data source is a file that can be shared among database users. A ODBC system data source is one that is local to a single computer. A ODBC user data source is only available to the user who created it.
41. Explain the three levels of transaction isolation supported by Oracle.
Answer: Oracle supports read committed, serializable, and read-only transaction isolation levels. Because of the way Oracle System Change Number (SCN) values are processed, Oracle never reads dirty data. Serializable isolation is possible, but the application program must be written to process the "Cannot serialize" exception. Applications can place locks explicitly using SELECT FOR UPDATE commands but this is not recommended.
42. Describe some reasons that someone familiar with Microsoft Access will want to learn VBA.
Answer: You can perform more complex functions and error handling can be accommodated by VBA. VBA code will execute faster since code runs faster than macros and maintenance is easier because VBA modules are stored with the forms and reports. Reading VBA is easier because you can see the entire code. Finally, you can pass parameters and use OLE automation better.
43. Under what conditions should indexes be used?
Answer: Indexes can be created to enforce uniqueness, to facilitate sorting, and to enable fast retrieval by column values. A good candidate for an index is a column that is frequently used with equal conditions in WHERE clauses.
44. Describe Website security issues.
Answer: Website security issues include unauthorized access to the several aspects of one's Website. Security measures should include all aspects of the system such as the network, operating level, database, and Web server. Regular monitoring and security testing by a company should help to avoid intrusion into one's system.
45. Describe the difference between embedded and dynamic SQL.
Answer: Embedded SQL is the process of including hard coded SQL statements. These statements do not change unless the source code is modified. Dynamic SQL is the process of generating SQL on the fly. The statements generated do not have to be the same each time.
46. Discuss the alternative terminology that is used in the relational model.
Answer: Relations are also called tables, and sometimes by the older data processing term files. A row is known as a tuple in the relational model, but may also be referred to as a record. Finally, relational model attributes are known as table columns and sometimes as fields.
47. List the three types of business rules and define each of them.
Answer: A derivation is a statement that is derived from other knowledge. A structured assertion is a statement that expresses some aspect of the static structure of an organization. An action assertion is a statement of a constraint on the actions of an organization.
48. Explain some of the main goals of normalization.
Answer: Normalization should minimize data redundancy. It should also simplify referential integrity constraints. Normalization will also make it easier to insert, update, and delete data. And finally, it provides better design.
49. What is Enterprise Resource Planning (ERP), and what kind of a database is used in an ERP application?
Answer: Enterprise Resource Planning (ERP) is an information system used in manufacturing companies and includes sales, inventory, production planning, purchasing and other business functions. An ERP system typically uses a multiuser database.
50. Why is modeling time-dependent data with a time stamp important?
Answer: The values of data may change. A time stamp helps to ensure that the previous value of the data stays in the database after it has changed so that you can see the before and after values through time. Without a time stamp, you will most likely lose some of the history.
51. Briefly describe the six database activities that occur during the systems development life cycle.
Answer: The enterprise modeling that analyzes the current data processing. Conceptual data modeling that identifies entities, relationships, and attributes. The logical database design that identifies data integrity and security requirements. The physical database design and definition that defines the database to a DBMS. The database implementation that installs and converts data from prior systems. Database maintenance that fixes errors in the database and database applications.
52. Describe and contrast a trigger and a procedure.
Answer: Triggers are stored and controlled in the DBMS. A trigger is executed automatically when a condition is met (INSERT, UPDATE, or DELETE). A procedure is also stored in a database. A procedure is not executed automatically.
53. You have been given a set of tables with data and asked to create a new database to store them. When you examine the data values in the tables, what are you looking for?
Answer: (1) Multivalued dependencies, (2) Functional dependencies, (3) Candidate keys, (4) Primary keys and (5) Foreign keys.
54. What is RFM analysis?
Answer: RFM analysis is a Business Intelligence (BI) reporting system that analyzes and ranks customers based on their purchasing patterns. R refers to "how recently" a customer placed an order, F refers to "how frequently" the customer orders, and M refers to "how much money" the customer spends. Typically, the customers are ranked into "20%" groups and assigned a number to represent their ranking. Thus 1 means top 20%, 2 the next 20% and so on. In this system a score of 1 is best and a score of 5 is worst. Thus a customer with an RFM score = 1 5 1 would be one who has ordered recently, does not order frequently, and who makes large purchases.
55. What is the difference between SQL Server complete and differential backups?
Answer: A complete backup makes a copy of the entire database. A differential backup makes a copy of the changes that have been made to the database since the last complete backup. A complete backup must be made before the first differential backup. Because differential backups are faster, they can be taken more frequently and the chance of data loss is reduced. Complete backups take longer but are slightly simpler to use for recovery.
56. What is the difference between a Java servlet and a Java applet?
Answer: An applet is a compiled Java bytecode program that is transmitted to a browser via HTTP and is invoked using the HTTP protocol on the user's workstation. A servlet is a Java program that is invoked on the server to respond to HTTP requests.
57. What is a cascading update?
Answer: Referential integrity constraints require that foreign key values in one table correspond to primary key values in another. If the value of the primary key is changed -- that is, updated -- the value of the foreign key must immediately be changed to match it. Cascading updates will set this change to be done automatically by the DBMS whenever necessary.
58. Why are functional dependencies not equations?
Answer: Equations deal with numerical relationships. A functional dependency deals with the existence of a determinant relationship between attributes, regardless of whether or not there is a numerical relationship between them. Thus, if we know that there is no hot water every Wednesday, No-Hot-Water is functionally dependent on Wednesday. So, if we know it is Wednesday, then we know we will have No-Hot-Water. This is a functional dependency, but not an equation.
59. Name and briefly describe the five SQL built-in functions.
Answer: COUNT: computes the number of rows in a table. SUM: totals numeric columns. AVG: computes the average value. MAX: obtains the maximum value of a column in a table. MIN: obtains the minimum value of a column in a table.
60. Explain the relationship between entity, entity class, and entity instance.
Answer: An entity is something that can be identified in the users' work environment, something that the users want to track. Entities of a given type are grouped into entity classes. An entity instance is the representation of a particular entity.
61. What is the relationship of ODBC, OLE DB, and ADO?
Answer: Developed first, the ODBC standard is for relational databases; while the OLE DB standard provides functionality for both relational and other databases. Finally, ADO was developed to provide easier access to OLE DB data for the non-object-oriented programmer.
62. What is deadlock? How can it be avoided? How can it be resolved once it occurs?
Answer: Deadlock occurs when two transactions are each waiting on a resource that the other transaction holds. Deadlock can be prevented by requiring transactions to acquire all locks at the same time; once it occurs, the only way to cure it is to abort one of the transactions and back out of partially completed work.
63. Explain the difference between the SQL Server simple, full, and bulk-logged recovery models.
Answer: With the simple recovery model, no logging is done. The only way to recover a database is to restore the database to the last backup. With full recovery, all database changes are logged. With bulk-logged database recovery, all changes are logged except those that cause large log entries.
64. Explain a join between tables
Answer: A join allows tables to be linked to other tables when a relationship between the tables exists. The relationships are established by using a common column in the tables and often uses the primary/foreign key relationship.
65. Why is the multivalue, multicolumn problem another form of the multivalued dependency problem?
Answer: Both problems try to store multiple values on an attribute in a table. In the multivalue, multiplecolumn problem, the multiple values are stored in different columns. In the mutlivalued dependency problem the multiple values are stored in different rows. In both cases, the solution is the same: store the multiple values in a separate table.
66. Describe JavaScript and VBScript.
Answer: JavaScript shares many features with Java. Developers use it to achieve interactivity. JavaScript is an open language and does not require a license. It is supported by both Internet Explorer and Netscape. VBScript is similar to JavaScript. It is based on Visual Basic but is simpler. Microsoft developed this language.
67. Explain the difference between an exclusive lock and a shared lock.
Answer: An exclusive lock prohibits other users from reading the locked resource; a shared lock allows other users to read the locked resource, but they cannot update it.
68. What is a SQL view? Briefly explain the use of views.
Answer: A SQL view is a virtual table built from other tables or views. Views are used to (1) hide columns or rows, (2) show the results of computed columns, (3) hide complicated SQL syntax, (4) layer built-in functions, (5) provide a level of indirection between application programs and tables, (6) assign different sets of processing permissions to tables, and (7) to assign different sets of triggers to the same table.
69. Explain the difference between total specialization and partial specialization.
Answer: Total specialization exists when every instance of a supertype must also be an instance of a subtype. Partial specialization exists when every instance of a supertype does not have to be an instance of a subtype.
70. Describe a data warehouse.
Answer: A data warehouse is organized around specific subjects. The data is defined using consistent naming, format, and encoding structure standards. The data contains a time element, so that the data can be studied for trends. No data in a data warehouse can be updated by end users.
71. Explain the systems development life cycle.
Answer: This is the traditional method to develop, maintain, and replace information systems that includes the project identification and selection, project initiation and planning, analysis, logical design, physical design, implementation, and maintenance steps. The process is a series of steps that are ideally completed in a linear fashion. In reality, the process is not linear and the process requires steps to be revisited and an overlap of steps.
72. List some of the chrematistics of good data definitions.
Answer: Definitions are gathered from the same sources and should be accompanied diagrams. A definition will include special conditions, examples, how the data is created, whether the data can change, who owns the data, whether the data is optional, and whether the data can be broken into something more atomic.
73. Explain how to add a NOT NULL column to a table.
Answer: First, add the column as NULL. Then use UPDATE to add data to every row. Finally use an ALTER TABLE . . . ALTER COLUMN statement to change the column constraint to NOT NULL.
74. Describe Web Services.
Answer: Web Services are improving the ability of computers to communicate over the Internet. These services use XML programs and usually run in the background. Easier integration of applications may be possible because developers do not need to be familiar with the technical details with applications that are being integrated. UDDI is a technical specification for creating a distributed registry of Web services and businesses that are open to communicating through Web services.
75. Describe and contrast SQL and QBE.
Answer: QBE is a direct-manipulation database language that uses a graphical approach to query construction. Some database systems translate QBE queries into SQL. QBE does not adhere to a standard but SQL does. Both SQL and QBE are relational database languages.
76. Describe concurrency control.
Answer: Concurrency control is the process managing simultaneous operations against a database so that database integrity is not compromised. There are two approaches to concurrency control. The pessimistic approach involves locking and the optimistic approach involves versioning.
77. Explain the "paradigm mismatch" between SQL and application programming languages.
Answer: SQL statements return a set of rows, while an application program works on one row at a time. To resolve this mismatch the results of SQL statements are processed as pseudofiles, using a cursor or pointer to specify which row is being processed.
78. Write an SQL SELECT statement to count the number of rows in STUDENT table and display the result with the label NumStudents.
Answer: SELECT COUNT(*) AS NumStudents FROM STUDENT;
79. What disadvantage of ODBC does OLE DB overcome?
Answer: By breaking the features and the functions of a DBMS into COM objects, OLE DB characteristic overcomes a major disadvantage of ODBC. With ODBC, a vendor must create an ODBC driver for almost all DBMS features and functions in order to participate in ODBC at all. This is a large task that requires a substantial initial investment. With OLE DB, however, a DBMS vendor can implement portions of their product.
80. What is the inconsistent values problem? Include an example not used in the text.
Answer: The inconsistent values problem occurs when different users or data sources use slightly different forms of the same data value. One example is where automobiles are specified as "Ford, 2-door, Red" in one cell and "Red Ford 2-door" in another.
81. What are to goals of OLE DB?
Answer: The major goals of OLE DB are to: (1) Create object interfaces for DBMS functionality pieces; (2) Increase flexibility for developers and users; (3) provide an object interface over any type of data; and (4) do not force data to be converted or moved from where it is.
82. What are the steps to follow when preparing to create a table?
Answer: 1. Identify the data type, length, and precision for each attribute. 2. Identify the columns that can accept a null value. 3. Identify the columns that need to be unique. 4. Identify primary and related foreign keys with the parent table being created before the child. 5. Determine default values. 6. Determine where the domain values are that need to be constrained. 7. Create the indexes.
83. Briefly describe an outer join.
Answer: An outer join includes the records that match and those that do not have a matching value in another table. Outer joins can be a LEFT outer join (includes all records from the first table listed) or a RIGHT outer join (includes all records from the second table listed). Outer joins are not easily used with more than two tables.
84. How are surrogate keys and metadata handled in MySQL?
Answer: MySQL uses integer data types combined with the property AUTO_INCREMENT to create surrogate keys. This creates a sequence that starts at one (1) and increases by one (1) for each new record. MySql maintain its metadata in a database named mysql. For example, this database maintains two tables named user and db.
85. Describe the differences between vertical and horizontal portioning.
Answer: Horizontal portioning is where the rows in a relation are separated by some criteria and placed into a new relation or file with the same layout as the original relation (in this case only the records in each file differ). Vertical portioning is where the columns in a relation are separated by some criteria and placed into a new relation or file with a different layout as the original relation.
86. In general, how are recursive relationships handled in a database design?
Answer: A recursive relationship is a relationship among entities of the same class, and is represented in the same way as other relationships are. The rows of the tables can take two different roles, however. Some are parent rows, and others are child rows. Further, the table will contain both its own primary key and the foreign key that links back to the table itself. If a row has no parent, then the value of the foreign key column in that row will be null. If the row has a parent, then there must be a foreign key value in that row that corresponds to the primary key value of another row in the table.
87. Provide an overview of XML.
Answer: XML is used to structure and manipulate data involved with a browser and is becoming the standard for e-commerce. XML uses tags that are similar to HTML in that they use the angle brackets, but XML describes the content whereas HTML describes the appearance. The XML schema standard was published in May 2001 by W3C.
88. Explain how a scenario is used for business rules.
Answer: A scenario is used to test business rules. It is a short script that describes how a business reacts to certain situations.
89. What is the multivalue, multicolumn problem? Include an example not used in the text.
Answer: The multivalue, multicolumn problem occurs when a table is designed to include multiple columns that hold variations of one type of attribute data. One example is where boat owners have the names of their boats stored as BOAT_01, BOAT_02 and BOAT_03.
90. Briefly explain an ERD.
Answer: An ERD is a detailed logical representation of the data for an organization. The ERD includes entities, attributes, relationships, and cardinalities. An ERD is the mechanism where an entity-relationship model is displayed.
91. Explain the difference between a dynamic and materialized view.
Answer: A dynamic view may be created every time that a specific view is requested by a user. A materialized view is created and or updated infrequently and it must be synchronized with its associated base table(s).
92. Describe the differences between a data warehouse and data mart.
Answer: A data warehouse is for the enterprise and contains multiple subjects. A data mart is for a specific functional area and focuses on one subject. A data warehouse is flexible and data-oriented and contains detailed historical data. A data mart is restrictive and project-oriented and contains some historical data.
93. Explain the difference between an ERD and EER.
Answer: An EER includes everything in an ERD and an EER allows for more complex relationships than an ERD. An EER allows for object-oriented data modeling and include supertypes and subtypes entities and inheritance.
94. What are some of the important security features of a DBMS?
Answer: One of the features includes the use of views which allows the presentation of only data needed by someone and limits the capability of database updates. The use of integrity controls includes such things as domains, assertions, and checks. Also authorization rules, user-defined procedures, encryption, authentication schemes, and backups are important.
95. Explain the difference between optimistic locking and pessimistic locking.
Answer: Optimistic locking assumes no transaction conflict will occur and deals with the consequences if it does. Pessimistic locking assumes that conflict will occur and so prevents it ahead of time with locks. In general, optimistic locking is preferred for the Internet and for many intranet applications.
96. Describe how a supertype/subtype relationship is mapped into a relation.
Answer: A separate relation is created for each supertype and subtype. The attributes common for all of the subtypes are assigned to the supertype. Each subtype has the primary key from the supertype assigned to it. A subtype discriminator is added to the supertype.
97. Explain snapshot replication.
Answer: Snapshot replication can be used when an application does not require that the data always be current. These applications can be updated by periodic table copying or snapshots. As part of the snapshot effort, all of the data to be included in the snapshot is collected at a primary point. Then a read-only snapshot is taken and the snapshot is sent to each site so that the update can be made.
98. What are the two means to describe the content of XML documents?
Answer: DTD (Document Type Declarations) and XML Schemas. An XML document that conforms to its DTD is called type-valid. A document can be well-formed and not be type-valid, either because it violates the structure of its DTD or because it has no DTD. However, DTDs have limitations and to overcome these limits XML Schemas were created. XML Schemas are XML documents that are the preferred method for defining document structure.
99. Briefly describe the three types of SQL commands.
Answer: Data definition language commands are used to create, alter, and drop tables. Data manipulation commands are used to insert, modify, update, and query data in the database. Data control language commands help the DBA to control the database.
100. Explain the role of metadata for the three-layer architecture.
Answer: Each of the three layers has a metadata layer linked with it. The metadata layer describes the properties or characteristics of the data. The operational metadata describe the data used in the various operational and external systems. The enterprise data warehouse metadata describe the reconciled data layer. The data mart metadata describes the derived data layer.
101. Who is E.F. Codd, and why is he significant in the development of modern database systems?
Answer: While working at IBM, E.F. Codd created the relational database model. A paper he published in 1970 presented his ideas to the world at large. His work is the foundation for most of the DBMSs currently in use, and thus forms the basis for database systems as we know and use them today.
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