Chapter 03 The Relational Database Model

Chapter 03: The Relational Database Model True / False 1. The practical significance of taking the logical view of a database is that it serves as a reminder of the simple file concept of data storage. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Moderate REFERENCES: 3-1 A Logical View of Data LEARNING OBJECTIVES: 03.01 - Describe the relational database model’s logical structure 2. You can think of a table as a persistent representation of a logical relation. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 3. The order of the rows and columns is important to the DBMS. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Moderate REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 4. Character data can contain any character or symbol intended for mathematical manipulation. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 5. Each table in a relational database must have a primary key. a. True b. Fals e ANSWER: True Page 1 Chapter 03: The Relational Database Model DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 6. The row’s range of permissible values is known as its domain. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 7. The idea of determination is unique to the database environment. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Moderate REFERENCES: 3-2a Dependencies LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 8. Only a single attribute, not multiple attributes, can define functional dependence. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Easy REFERENCES: 3-2a Dependencies LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 9. If the attribute (B) is functionally dependent on a composite key (A) but not on any subset of that composite key, the attribute (B) is fully functionally dependent on (A). a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Moderate REFERENCES: 3-2a Dependencies LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 10. A null is created when you press the Enter key or the Tab key to move to the next entry without making a prior entry Page 2 Chapter 03: The Relational Database Model of any kind. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-2b Types of Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 11. Depending on the sophistication of the application development software, nulls can create problems when functions such as COUNT, AVERAGE, and SUM are used. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-2b Types of Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 12. RDBMSs enforce integrity rules automatically. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-3 Integrity Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 13. Relational algebra defines the theoretical way of manipulating table contents using relational operators. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-4 Relational Algebra LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 14. The SELECT operator yields a vertical subset of a table. a. True b. Fals e ANSWER: False Page 3 Chapter 03: The Relational Database Model DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 15. The DIFFERENCE operator subtracts one table from the other. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 16. In a natural join, the column on which the join was made occurs twice in the new table. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 17. The DIVIDE operation uses one single-column table (e.g., column “a”) as the divisor and one two-column table (e.g., columns “a” and “b”) as the dividend. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 18. A data dictionary is sometimes described as “the database designer’s database” because it records the design decisions about tables and their structures. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog Page 4 Chapter 03: The Relational Database Model 19. The one-to-many (1:M) relationship is easily implemented in the relational model by putting the foreign key of the “1” side in the table of the “many” side as a primary key. a. True b. Fals e ANSWER: False DIFFICULTY: Difficulty: Moderate REFERENCES: 3-6a The 1:M Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 20. As rare as 1:1 relationships should be, certain conditions absolutely require their use. a. True b. Fals e ANSWER: True DIFFICULTY: Difficulty: Easy REFERENCES: 3-6b The 1:1 Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model Multiple Choice 21. _____ logic, used extensively in mathematics, provides a framework in which an assertion (statement of fact) can be verified as either true or false. a. Predicate b. Database d. Index c. Relationa l ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-1 A Logical View of Data LEARNING OBJECTIVES: 03.01 - Describe the relational database model’s logical structure 22. Each table _____ represents an attribute. a. column b. row c. dimension d. valu e ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 23. Date attributes contain calendar dates stored in a special format known as the _____ date format. a. Epoch b. calendar c. Julian d. logical ANSWER: c Page 5 Chapter 03: The Relational Database Model DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 24. In the relational model, _____ are important because they are used to ensure that each row in a table is uniquely identifiable. a. relations b. keys c. indexes d. logical structures ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-2 Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 25. A _____ is any key that uniquely identifies each row. a. superkey b. special key d. candidate key c. foreign key ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-2b Types of Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 26. A _____ key can be described as a minimal superkey, a superkey without any unnecessary attributes. a. secondary b. candidate c. primary d. foreign ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-2b Types of Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 27. A _____ is the primary key of one table that has been placed into another table to create a common attribute. a. superkey b. composite primary key c. candidate d. foreign key key ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-2b Types of Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 28. A _____ key is defined as a key that is used strictly for data retrieval purposes. Page 6 Chapter 03: The Relational Database Model a. lookup b. foreign c. candidate d. secondary ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-2b Types of Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 29. Referential _____ dictates that the foreign key must contain values that match the primary key in the related table, or must contain null. b. uniqueness a. integrit y c. model d. attribute ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-3 Integrity Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 30. The CUSTOMER table’s primary key is CUS_CODE. The CUSTOMER primary key column has no null entries, and all entries are unique. This is an example of _____ integrity. a. entity b. referentia l c. relational d. null ANSWER: a DIFFICULTY: Difficulty: Moderate REFERENCES: 3-3 Integrity Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 31. The _____ constraint can be placed on a column to ensure that every row in the table has a value for that column. a. UNIQUE b. NOT NULL c. VALUE d. EMPTY ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-3 Integrity Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 32. To be considered minimally relational, the DBMS must support the key relational operators _____, PROJECT, and JOIN. a. INTERSECT b. UNION c. DIFFERENCE d. SELECT ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-4 Relational Algebra Page 7 Chapter 03: The Relational Database Model LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 33. _____, also known as RESTRICT, yields values for all rows found in a table that satisfy a given condition. a. INTERSECT b. UNION c. DIFFERENCE d. SELECT ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 34. _____ returns only the attributes requested, in the order in which they are requested. a. PROJECT b. SELECT c. UNION d. DIFFERENCE ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 35. When two or more tables share the same number of columns, and when their corresponding columns share the same or compatible domains, they are said to be _____. a. intersect-compatible b. union-compatible c. difference-compatible d. select-compatible ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 36. A(n) _____ join links tables by selecting only the rows with common values in their common attribute(s). b. unique a. attribut e c. foreign d. natural ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 37. _____ are especially useful when you are trying to determine what values in related tables cause referential integrity problems. b. Outer joins a. Inner joins c. Equijoins d. Theta joins Page 8 Chapter 03: The Relational Database Model ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 38. A(n) _____ only returns matched records from the tables that are being joined. b. inner a. outer join join c. equijoin d. theta join ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 39. A _____ contains at least all of the attribute names and characteristics for each table in the system. a. data dictionary b. relational schema c. logical schema d. database ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog 40. The _____ is actually a system-created database whose tables store the user/designer-created database characteristics and contents. b. systematic a. database tuple database c. unique index d. system catalog ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog 41. In a database context, the word _____ indicates the use of the same attribute name to label different attributes. a. redundanc b. homonym y c. duplicate d. synonym ANSWER: b DIFFICULTY: Difficulty: Moderate REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog Page 9 Chapter 03: The Relational Database Model 42. In a database context, a(n) _____ indicates the use of different names to describe the same attribute. a. entity b. duplicate c. synonym d. homonym ANSWER: c DIFFICULTY: Difficulty: Easy REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog 43. The _____ relationship is the “relational model ideal.” a. 1:1 b. 1:M c. M:1 d. M:N ANSWER: b DIFFICULTY: Difficulty: Easy REFERENCES: 3-6 Relationships within the Relational Database LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 44. The _____ relationship should be rare in any relational database design. a. 1:1 b. 1:M c. M:1 d. M:N ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-6 Relationships within the Relational Database LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 45. _____ relationships can be implemented by creating a new entity in 1:M relationships with the original entities. a. 1:N b. M:1 d. 1:1 c. M: N ANSWER: c DIFFICULTY: Difficulty: Easy REFERENCES: 3-6c The M:N Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 46. Another name for a composite entity is a(n) _____ entity. a. bridge b. linked c. directive d. associative ANSWER: a DIFFICULTY: Difficulty: Easy REFERENCES: 3-6c The M:N Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model Page 10 Chapter 03: The Relational Database Model 47. A(n) _____ is an orderly arrangement used to logically access rows in a table. b. superke a. primary rule y c. relationship d. index ANSWER: d DIFFICULTY: Difficulty: Easy REFERENCES: 3-8 Indexes LEARNING OBJECTIVES: 03.07 - Explain the purpose of indexing in a relational database 48. When you define a table’s primary key, the DBMS automatically creates a(n) _____ index on the primary key column(s) you declared. a. key b. composite d. primary c. uniqu e ANSWER: c DIFFICULTY: Difficulty: Easy REFERENCES: 3-8 Indexes LEARNING OBJECTIVES: 03.07 - Explain the purpose of indexing in a relational database 49. According to Codd’s _____ rule of relational databases, application programs and ad hoc facilities are logically unaffected when changes are made to the table structures that preserve the original table values (changing order of columns or inserting columns). a. distribution independence b. logical data independence d. integrity independence c. comprehensive data sublanguage ANSWER: b DIFFICULTY: Difficulty: Moderate REFERENCES: 3-9 Codd’s Relational Database Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 50. According to Codd's _____ rule of relational databases, if the system supports low-level access to the data, users must not be allowed to bypass the integrity rules of the database. a. nonsubversion b. information d. view updating c. guaranteed access ANSWER: a DIFFICULTY: Difficulty: Moderate REFERENCES: 3-9 Codd’s Relational Database Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table Completion 51. A table is also called a(n) _____ because the relational model’s creator, E. F. Codd, used the two terms as synonyms. ANSWER: relation Page 11 Chapter 03: The Relational Database Model DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 52. In a relational table, each column has a specific range of values known as the _____ domain. ANSWER: attribute DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 53. A primary key is a(n) _____ key chosen to be the primary means by which rows of a table are uniquely identified. ANSWER: candidate DIFFICULTY: Difficulty: Easy REFERENCES: 3-1a Tables and Their Characteristics LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 54. In a relational model, _____ are used to establish relationships among tables and to ensure the integrity of the data. ANSWER: keys DIFFICULTY: Difficulty: Easy REFERENCES: 3-2 Keys LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 55. To avoid nulls, some designers use special codes, known as _____, to indicate the absence of some value. ANSWER: flags DIFFICULTY: Difficulty: Easy REFERENCES: 3-3 Integrity Rules LEARNING OBJ03.02 - Identify the relational model’s basic components and explain the ECTIVES: structure, contents, and characteristics of a relational table 56. The relational operators have the property of _____; that is, the use of relational algebra operators on existing relations (tables) produces new relations. ANSWER: closure DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 57. PRODUCT yields all possible pairs of rows from two tables, also known as the _____ product. ANSWER: Cartesian DIFFICULTY: Difficulty: Moderate REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents Page 12 Chapter 03: The Relational Database Model 58. _____ is the real power behind the relational database, allowing the use of independent tables linked by common attributes. ANSWER: JOIN DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 59. A(n) _____ links tables on the basis of an equality condition that compares specified columns of each table. ANSWER: equijoin DIFFICULTY: Difficulty: Easy REFERENCES: 3-4b Relational Set Operators LEARNING OBJECTIVE03.03 - Use relational database operators to manipulate relational table S: contents 60. A(n) ______ provides a detailed description of all tables found within the user/designer-created database. ANSWER: data dictionary DIFFICULTY: Difficulty: Easy REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog 61. The _____ catalog can be described as a detailed system data dictionary that describes all objects within the database, including data about table names, the table’s creator and creation date, the number of columns in each table, the data type corresponding to each column, index filenames, index creators, authorized users, and access privileges. ANSWER: system DIFFICULTY: Difficulty: Easy REFERENCES: 3-5 The Data Dictionary and the System Catalog LEARNING OBJECTIV03.04 - Explain the purpose and components of the data dictionary and ES: system catalog 62. The _____ relationship is the relational database norm. ANSWER: 1:M DIFFICULTY: Difficulty: Moderate REFERENCES: 3-6a The 1:M Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 63. _____ relationships cannot be implemented as such in the relational model. ANSWER: M:N DIFFICULTY: Difficulty: Moderate REFERENCES: 3-6c The M:N Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 64. If one department chair—a professor—can chair only one department, and one department can have only one Page 13 Chapter 03: The Relational Database Model department chair. The entities PROFESSOR and DEPARTMENT exhibit a(n) _____ relationship. ANSWER: 1:1 DIFFICULTY: Difficulty: Moderate REFERENCES: 3-6b The 1:1 Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 65. One characteristic of generalization hierarchies is that they are implemented as _____ relationships. ANSWER: 1:1 DIFFICULTY: Difficulty: Moderate REFERENCES: 3-6b The 1:1 Relationship LEARNING OBJE 03.05 - Identify appropriate entities and then the relationships among the CTIVES: entities in the relational database model 66. The proper use of _____ keys is crucial to controlling data redundancy. ANSWER: foreign DIFFICULTY: Difficulty: Easy REFERENCES: 3-7 Data Redundancy Revisited LEARNING OBJECTIV 03.06 - Describe how data redundancy is handled in the relational ES: database model 67. Proper data _____ design requires carefully defined and controlled data redundancies to function properly. ANSWER: warehousing DIFFICULTY: Difficulty: Easy REFERENCES: 3-7 Data Redundancy Revisited LEARNING OBJECTIV 03.06 - Describe how data redundancy is handled in the relational ES: database model 68. A(n) _____ index is an index in which the index key can have only one pointer value (row) associated with it. ANSWER: unique DIFFICULTY: Difficulty: Easy REFERENCES: 3-8 Indexes LEARNING OBJECTIVES: 03.07 - Explain the purpose of indexing in a relational database 69. An index key can have multiple _____ (a composite index). ANSWER: attributes DIFFICULTY: Difficulty: Easy REFERENCES: 3-8 Indexes LEARNING OBJECTIVES: 03.07 - Explain the purpose of indexing in a relational database 70. Dr. Codd’s _____ rule of relational database states that every value in a table is guaranteed to be accessible through a combination of table name, primary key value, and column name. ANSWER: guaranteed Access DIFFICULTY: Difficulty: Moderate REFERENCES: 3-9 Codd’s Relational Database Rules LEARNING OBJ 03.02 - Identify the relational model’s basic components and explain the Page 14 Chapter 03: The Relational Database Model ECTIVES: structure, contents, and characteristics of a relational table Essay 71. What is a key and how is it important in a relational model? ANSWER: In a relational model, keys are important because they are used to ensure that each row in a table is uniquely identifiable. They are also used to establish relationships among tables and to ensure the integrity of the data. A key consists of one or more attributes that determine other attributes. For example, an invoice number identifies all of the invoice attributes, such as the invoice date and the customer name. DIFFICULTY: Difficulty: Moderate REFERENCES 3-2 Keys : LEARNING O 03.02 - Identify the relational model’s basic components and explain the BJECTIVES: structure, contents, and characteristics of a relational table 72. Define entity integrity. What are the two requirements to ensure entity integrity? ANSWER: Entity integrity is the condition in which each row (entity instance) in the table has its own unique identity. To ensure entity integrity, the primary key has two requirements: (1) all of the values in the primary key must be unique. (2) no key attribute in the primary key can contain a null. DIFFICULTY: Difficulty: Moderate REFERENCES 3-2b Types of Keys : LEARNING O 03.02 - Identify the relational model’s basic components and explain the BJECTIVES: structure, contents, and characteristics of a relational table 73. Describe the use of null values in a database. ANSWER: Null values are problematic in a relational model. A null is the absence of any data value, and it is never allowed in any part of the primary key. From a theoretical perspective, it can be argued that a table that contains a null is not properly a relational table at all. From a practical perspective, however, some nulls cannot be reasonably avoided. For example, not all students have a middle initial. As a general rule, nulls should be avoided as much as reasonably possible. In fact, an abundance of nulls is often a sign of a poor design. Also, nulls should be avoided in the database because their meaning is not always identifiable. For example, a null could represent: • An unknown attribute value. • A known, but missing, attribute value. • A “not applicable” condition. DIFFICULTY: Difficulty: Moderate REFERENCES 3-2b Types of Keys : LEARNING O 03.02 - Identify the relational model’s basic components and explain the BJECTIVES: structure, contents, and characteristics of a relational table 74. Describe the use of the INTERSECT operator. ANSWER: INTERSECT yields only the rows that appear in both tables. As with UNION, the Page 15 Chapter 03: The Relational Database Model tables must be union-compatible to yield valid results. For example, you cannot use INTERSECT if one of the attributes is numeric and one is character-based. For the rows to be considered the same in both tables and appear in the result of the INTERSECT, the entire rows must be exact duplicates. DIFFICULTY: Difficulty: Moderate REFERENCES 3-4b Relational Set Operators : LEARNING O 03.03 - Use relational database operators to manipulate relational table contents BJECTIVES: 75. Define an index. Explain the role of indexes in a relational database. ANSWER: An index is an orderly arrangement used to logically access rows in a table. From a conceptual point of view, an index is composed of an index key and a set of pointers. The index key is, in effect, the index’s reference point. More formally, an index is an ordered arrangement of keys and pointers. Each key points to the location of the data identified by the key. DBMSs use indexes for many different purposes. An index can be used to retrieve data more efficiently. Indexes can also be used by a DBMS to retrieve data ordered by a specific attribute or attributes. For example, creating an index on a customer’s last name will allow you to retrieve the customer data alphabetically by the customer’s last name. Also, an index key can be composed of one or more attributes. Indexes play an important role in DBMSs for the implementation of primary keys. When you define a table’s primary key, the DBMS automatically creates a unique index on the primary key column(s) you declared. DIFFICULTY: Difficulty: Moderate REFERENCES 3-8 Indexes : LEARNING O 03.07 - Explain the purpose of indexing in a relational database BJECTIVES: Page 16