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Unformatted text preview: CHAPTER 7: QUESTIONS AND ANSWERS 1. What is an insertion anomaly? Ans: An insertion anomaly occurs when extra data beyond the desired data must be added to a table. 2. What is an update anomaly? Ans: An update anomaly occurs when it is necessary to change multiple rows to modify only a single fact. 3. What is a deletion anomaly? Ans: A deletion anomaly occurs whenever deleting a row inadvertently causes other data to be deleted. 4. What is the cause of modification anomalies? Ans: Poor database design causes the modification anomaly. A good database design avoids modification anomalies by eliminating excessive redundancies. 5. What is a functional dependency? Ans: A functional dependency is a constraint about two or more columns of a table. X determines Y (X → Y) if there exists at most one value of Y for every value of X. 6. How is a functional dependency like a candidate key? Ans: You can think about functional dependency as identifying potential primary keys. By stating that X → Y, if X and Y are placed together in a table without other columns, X is a candidate key. Every determinant (LHS) is a potential primary key if placed in a table with the other columns that it determines. 7. Can a software design tool identify functional dependencies? Briefly explain your answer. Ans: No, a software design tool cannot identify functional dependencies. Functional dependencies must be asserted during the database development process. Typically, the database designer interacts with users to understand the functional dependencies that exist for a table. Software design tools can aid a designer by eliminating FDs that do not exist and by suggesting FDs that are not contradicted by the data. Design tools examine sample rows in a table to see what functional dependencies do not hold. There are several commercial database design tools that automate the process of eliminating dependencies through examination of sample rows. Ultimately, the database designer must make the final decision about what functional dependencies exist in a table. 8. What is the meaning of an FD with multiple columns on the right-hand side? Ans: Multiple columns on the RHS abbreviate separate FDs with the LHS determining each of the RHS columns. 9. Why should you be careful when writing FDs with multiple columns on the left-hand side? Ans: Multiple columns on the LHS indicate that the combination of columns determines the RHS column(s). You should be careful that you have written the LHS with the minimal columns for the dependency. Otherwise, the normalization rules and software procedures may not work as intended. 10. What is a normal form? Ans: A normal form is a rule about allowable dependencies. Each normal form removes certain kinds of redundancies....
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- Spring '11
- Database normalization, simple synthesis procedure