Chapter 3 The Relational Database Model

Chapter 3: The Relational Database Model Key 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. TRUE 2. You can think of a table as a persistent representation of a logical relation. 3. Because the relational model uses attribute values to establish relationships among tables, many database users correctly assume that the term relation refers to such relationships. TRUE FALSE 4. The order of the rows and columns is important to the DBMS. FALSE 5. Numeric data are data on which you can perform meaningful arithmetic procedures. 6. Character data can contain any character or symbol intended for mathematical manipulation. TRUE FALSE 7. Each table in a relational database must have a primary key. 8. A proper understanding of the concept and use of keys in a relational database model is very important. 9. In a relational model, if A determines B, C, and D, you write A = B, C, D. TRUE TRUE FALSE 10. Only a single attribute, not multiple attributes, can define functional dependence. 11. 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). 12. 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 of any kind. FALSE TRUE TRUE 1 13. There is never a good reason to use null values in a database. 14. Depending on the sophistication of the application development software, nulls can create problems when functions such as COUNT, AVERAGE, and SUM are used. FALSE TRUE 15. Tables within a database share common attributes that enable the tables to be linked together. TRUE 16. A foreign key must exist in both tables that have a relationship. 17. All RDBMSs enforce integrity rules automatically. FALSE FALSE 18. Referential and entity integrity are two names for the same thing. 19. The SELECT operator yields a vertical subset of a table. FALSE FALSE 20. Tables must have the same attribute characteristics (the columns and domains must be compatible) to be used in a UNION. TRUE 21. The DIFFERENCE operator subtracts one table from the other. 22. In a natural join, the column on which the join was made occurs twice in the new table. TRUE FALSE 23. A left outer joinon tables CUSTOMER and AGENT yields all of the rows in the CUSTOMER table, including those that do not have a matching value in the AGENT table TRUE 24. The DIVIDE operation uses one single-column table (i.e. column “a”) as the divisor and one two-column table (i.e. columns “a” and “b”) as the dividend. TRUE 25. A data dictionary contains metadata—data about data. TRUE 2 26. A data dictionary is sometimes described as “the database designer’s database” because it records the design decisions about tables and their structures. 27. Current relational database software generally provides only a system catalog (and not a data dictionary). TRUE TRUE 28. 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. FALSE 29. As rare as 1:1 relationships should be, certain conditions absolutely requiretheir use. 30. DBMSs use indexes for many different purposes. TRUE TRUE 31. ____ logic, used extensively in mathematics, provides a framework in which an assertion (statement of fact) can be verified as either true or false. A. B. C. D. 32. The relational database model enables you to view data ____ rather than ____. A. B. C. D. 33. index key table relationship A(n) ____ is perceived as a two-dimensional structure composed of rows and columns. A. B. C. D. 35. relationally, hierarchically hierarchically, relationally physically, logically logically, physically The relational model’s creator, E. F. Codd, used the term relation as a synonym for ____. A. B. C. D. 34. Predicate Database Relational Index table rowset attribute intersection Date attributes contain calendar dates stored in a special format known as the ____ date format. A. B. C. D. Epoch Calendar Julian logical 3 36. ____ data can have only a true or false (yes or no) values. A. B. C. D. 37. In the relational model, ____ are important because they are used to ensure that each row in a table is uniquely identifiable A. B. C. D. 38. foreign unique composite complete A ____ is any key that uniquely identifies each row. A. B. C. D. 43. key attribute logical attribute key determiner selector If the attribute (B) is functionally dependent on a composite key (A) but not on any subset of that ____ key, the attribute (B) is fully functionally dependent on (A). A. B. C. D. 42. logically dependent on owned by determined by functionally dependent on Any attribute that is part of a key is known as a ____. A. B. C. D. 41. contains is related to owns determines The attribute B is ____ the attribute A if each value in column A determines one and only one value in column B. A. B. C. D. 40. relations keys indexes logical structures In the context of a database table, the statement “A ____ B” indicates that if you know the value of attribute A, you can look up the value of attribute B. A. B. C. D. 39. Logical Character Date Numeric superkey special selective candidate No data entry at all is known as a(n) ____. A. B. C. D. nil null empty zero 4 44. Controlled ____ makes a relational database work. A. B. C. D. 45. A ____ is a textual representation of the database tables where each table is listed by its name followed by the list of its attributes in parentheses. A. B. C. D. 46. lookup foreign candidate secondary A ____ key can be described as a superkey without unnecessary attributes, that is, a minimal superkey. A. B. C. D. 50. integrity uniqueness direction relations A ____ key is defined as a key that is used strictly for data retrieval purposes. A. B. C. D. 49. PROD_DESCRIPT PROD_CODE PROD_PRICE PROD_ON_HAND Referential ____ means that if the foreign key contains a value, that value refers to an existing valid tuple (row) in another relation. A. B. C. D. 48. relational dictionary logical schema relational schema data dictionary In the following table description, ____ is the primary key. PRODUCT (PROD_CODE, PROD_DESCRIPT, PROD_PRICE, PROD_ON_HAND, VEND_CODE) A. B. C. D. 47. relations logic chaos redundancy secondary candidate primary foreign All primary key entries are unique, and no part of a primary key may be ____. A. B. C. D. zero a foreign key null a candidate key 5 51. A 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. B. C. D. 52. The ____ constraint can be placed on a column to ensure that every row in the table has a value for that column. A. B. C. D. 53. PROJECT SELECT UNION DIFFERENCE ____ combines all rows from two tables, excluding duplicate rows. A. B. C. D. 57. INTERSECT UNION DIFFERENCE SELECT ____ yields a vertical subset of a table. A. B. C. D. 56. INTERSECT UNION DIFFERENCE SELECT ____, also known as RESTRICT, yields values for all rows found in a table that satisfy a given condition. A. B. C. D. 55. HAS VALUE NOT NULL MUST HAVE VALUE NOT EMPTY To be considered minimally relational, the DBMS must support the key relational operators ____, PROJECT, and JOIN. A. B. C. D. 54. entity referential complete null INTERSECT UNION DIFFERENCE SELECT ____ yields only the rows that appear in both tables. A. B. C. D. INTERSECT UNION DIFFERENCE SELECT 6 58. A(n) ____ join links tables by selecting only the rows with common values in their common attribute(s). A. B. C. D. 59. The equijoin takes its name from the comparison operator ____ used in the condition. A. B. C. D. 60. redundancy homonym duplicate synonym In a database context, a(n) ____indicates the use of different names to describe the same attribute. A. B. C. D. 65. meta dictionary schema data dictionary system catalog In a database context, the word ____indicates the use of the same attribute name to label different attributes. A. B. C. D. 64. data dictionary relational schema logical schema join The ____ is actually a system-created database whose tables store the user/designer-created database characteristics and contents. A. B. C. D. 63. in another table null out of the results with matching values from the original table A ____ contains at least all of the attribute names and characteristics for each table in the system. A. B. C. D. 62. * < = > In an outer join, the matched pairs would be retained and any unmatched values in the other table would be left ____. A. B. C. D. 61. equal unique foreign natural entity duplicate synonym homonym ____ relational type is the “relational model ideal.” A. B. C. D. 1:1 1:M M:1 M:N 7 66. The ____ relationship should be rare in any relational database design. A. B. C. D. 67. Since it is used to link the tables that originally were related in a M:N relationship, the composite entity structure includes—as foreign keys—at least the ____ keys of the tables that are to be linked. A. B. C. D. 68. table superkey relationship index When you define a table’s primary key, the DBMS automatically creates a(n) ____ index on the primary key column(s) you declared. A. B. C. D. 70. composite super primary unique A(n) ____ is an ordered arrangement of keys and pointers. A. B. C. D. 69. 1:1 1:M M:1 M:N key incomplete unique primary Codd’s rule of ____ states: 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. B. C. D. Nonsubversion Logical Data Independence Comprehensive Data Sublanguage Integrity Independence 71. The logical view of the relational database is facilitated by the creation of data relationships based on a logical construct known as a(n) ____________________. 72. In a relational table, each column has a specific range of values known as the ____________________ domain. relation attribute 73. In the relational model, ____________________ are important because they are used to ensure that each row in a table is uniquely identifiable. keys 74. Attribute A determines attribute B (that is, B is functionally ____________________ on A) if all of the rows in the table that agree in value for attribute A also agree in value for attribute B. dependent 8 75. A(n) ____________________ keycan be described as a superkey without unnecessary attributes. 76. If the foreign key contains either matching values or nulls, the table that makes use of that foreign key is said to exhibit ____________________ integrity candidate referential 77. The following example exhibits ____________________ integrity: The CUSTOMER table’s primary key is CUS_CODE. The CUSTOMER primary key column has no null entries, and all entries are unique. Similarly, the AGENT table’s primary key is AGENT_CODE, and this primary key column also is free of null entries. entity 78. To avoid nulls, some designers use special codes, known as ____________________, to indicate the absence of some value. flags 79. The relational operators have the property of ____________________; that is, the use of relational algebra operators on existing relations (tables) produces new relations. closure 80. PRODUCT yields all possible pairs of rows from two tables—also known as the ____________________ product. 81. ____________________ is the real power behind the relational database, allowing the use of independent tables linked by common attributes. Cartesian JOIN 82. A(n) ____________________ links tables on the basis of an equality condition that compares specified columns of each table. equijoin 83. ____________________ joins are especially useful when you are trying to determine what value(s) in related tables cause(s) referential integrity problems. Outer 84. A(n) ____________________provides a detailed description of all tables found within the user/designer-created database. 85. 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. data dictionary system 9 86. The ____________________ relationship is the relational database norm. 87. If one department chair—a professor—can chair only one department and one department can have only one department chair, then the entities PROFESSOR and DEPARTMENT exhibit a(n) ____________________ relationship. 1:M 1:1 88. ____________________ relationships can be implemented by creating a new entity in 1:M relationships with the original entities. M:N 89. Fortunately, the problems inherent in the ____________________ relationship can easily be avoided by creating a composite entity. many-to-many or M:N 90. The proper use of ____________________ keys is crucial to controlling data redundancy. 91. Proper data ____________________ design requires carefully defined and controlled data redundancies to function properly. 92. A(n) ____________________is an orderly arrangement used to logically access rows in a table. foreign warehousing index 93. A(n) ____________________ index is an index in which the index key can have only one pointer value (row) associated with it. unique 94. The index key can have multiple attributes, this is called a(n) ____________________ index. composite 95. Codd’s rule of ____________________ states that every value in a table is guaranteed to be accessible through a combination of table name, primary key value, and column name. Guaranteed Access 10 96. What are the characteristics of a relational table? 1 A table is perceived as a two-dimensional structure composed of rows and columns. 2 Each table row (tuple) represents a single entity occurrence within the entity set. 3 Each table column represents an attribute, and each column has a distinct name. 4 Each row/column intersection represents a single data value. 5 All values in a column must conform to the same data format. 6 Each column has a specific range of values known as the attribute domain. 7 The order of the rows and columns is immaterial to the DBMS. 8 Each table must have an attribute or a combination of attributes that uniquely identifies each row. 97. What is a key and why is it important in the relational model? In the 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. Therefore, a proper understanding of the concept and use of keys in the relational model is very important. A keyconsists 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. 98. Describe the use of nulls in a database. Nulls can never be part of a primary key, and they should be avoided—to the greatest extent possible—in other attributes, too. There are rare cases in which nulls cannot be reasonably avoided when you are working with nonkey attributes. For example, one of an EMPLOYEE table’s attributes is likely to be the EMP_INITIAL. However, some employees do not have a middle initial. Therefore, some of the EMP_INITIAL values may be null. There may be situations in which a null exists because of the nature of the relationship between two entities. In any case, even if nulls cannot always be avoided, they must be used sparingly. In fact, the existence of nulls in a table is often an indication of poor database design. 99. Describe the use of the UNION operator. UNION combines all rows from two tables, excluding duplicate rows. The tables must have the same attribute characteristics (the columns and domains must be compatible) to be used in the UNION. When two or more tables share the same number of columns, when their corresponding columns share the same (or compatible) domains, they are said to be union-compatible. 11 100. What is the system catalog? Like the data dictionary, the system catalog contains metadata. The system 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. Because the system catalog contains all required data dictionary information, the terms system catalog and data dictionary are often used interchangeably. In fact, current relational database software generally provides only a system catalog, from which the designer’s data dictionary information may be derived. The system catalog is actually a system-created database whose tables store the user/designer-created database characteristics and contents. Therefore, the system catalog tables can be queried just like any user/designer-created table. 12