# chap007 - Chapter 7 Information Retrieval from Relational...

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Chapter 7 Information Retrieval from Relational Databases Review Questions R1. Which of the relational algebra operators is needed to retrieve a vertical subset (i.e., a subset of columns) from a relational database table? PROJECT R2. Which of the relational algebra operators is needed to retrieve a horizontal subset (i.e., a subset of rows) from a relational database table? SELECT R3. Which of the relational algebra operators is needed to combine two tables together in a query? JOIN R4. What is the standard format of a SQL query statement? Select ______ From______ Where_______; R5. Which component of a SQL statement accomplishes a relational algebra SELECT operation? The “Where” clause. R6. Which component of a SQL statement accomplishes a relational algebra PROJECT operation? The “Select” clause. R7. Which components of a SQL statement accomplish a relational algebra JOIN operation? Solutions Manual to accompany Dunn, Enterprise Information Systems: A Pattern Based Approach, 3e 89

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Chapter 7 The “From” clause specifies which tables are to be joined; the “Where” clause specifies the attribute(s) on which the tables are to be joined. R8. What is the advantage to creating a query that includes a date constraint as a parameter query? Each time the user needs to run the query for different dates, the user does not need to change the query design; but rather the user is prompted to enter the desired date upon running the query. R9. What is the difference between an aggregation (vertical calculation) and a horizontal calculation? An aggregation (vertical calculation) combines numbers within a single column into a calculation such as a sum, an average, a count, etc. A horizontal calculation combines numbers across columns within a single row. R10.Give an example of a query for which you would need to use a left join instead of an inner join. Left joins keep unmatched values from the left table and fill in the corresponding information from the right table for those left table values that do have matches. Thus any example of joining two tables for which some values of the left table will have matches in the right table and some won’t is appropriate. One example is a join of Employees and Training Courses for which not all employees have yet taken training courses. Another example is a join of Library Book Borrowing and Library Book Return events, as some book borrowings will not yet have resulted in returns. 90 Solutions Manual to accompany Dunn, Enterprise Information Systems: A Pattern Based Approach, 3e
Information Retrieval from Relational Databases Discussion Questions D1. What are the advantages (and/or disadvantages) of SQL compared to Relational Algebra? SQL allows multiple relational algebra operations to be performed in a single

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chap007 - Chapter 7 Information Retrieval from Relational...

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