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Email > StdCity, StdClass You should not write StdSSN, Email > StdCity, StdClass be
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cause these F D s imply that the combination o f StdSSN and Email is the determinant Thus, y o u should write F D s so that the LHS does not contain unneeded c o l u m n s The prohibition against unneeded columns for determinants is the same as the prohibition against unneeded columns in candidate keys Both determinants and candidate keys must be minimal
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using sample data to eliminate potential FDs If two rows have the same value for the LHS but different values for the RHS, an FD cannot exist Some commercial normalization programs use this technique to help a user determine FDs
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A functional dependency cannot be proven to exist by examining the rows o f a table H o w ever, y o u can falsify a functional dependency (ie, prove that a functional dependency does not exist) by examining the contents o f a table For example, in the university database (Table 71) w e can conclude that StdClass does not determine StdCity because there are two rows with the same value for StdClass but a different value for StdCity Thus, it is sometimes helpful to examine sample rows in a table to eliminate potential functional dependencies There are several commercial database design tools that automate the process o f eliminat ing dependencies through examination o f sample rows Ultimately, the database designer must make the final decision about the functional dependencies that exist in a table
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Normalization is the process o f removing redundancy in a table so that the table is easier to modify A number o f normal forms have been developed to remove redundancies A normal form is a rule about allowable dependencies Each normal form removes certain kinds o f redundancies A s shown in Figure 7 3 , first normal form ( I N F ) is the starting point A l l tables without repeating groups are in INF 2 N F is stronger than INF Only a subset o f the I N F tables is in 2NF Each successive normal form refines the previous normal form to remove additional kinds o f redundancies Because B C N F ( B o y c e - C o d d Normal Form) is a revised (and stronger) definition for 3NF, 3 N F and B C N F are shown in the same part o f Figure 73 2 N F and 3 N F / B C N F are rules about functional dependencies If the functional depen dencies for a table match the specified pattern, the table is in the specified normal form 3 N F / B C N F is the most important in practice because higher normal forms involve other kinds o f dependencies that are less c o m m o n and more difficult to understand Therefore, most emphasis is given to 3NF/BCNF Section 73 presents 4 N F as a way to reason about M-way relationships Section 74 presents 5 N F and D K N F (domain key normal form) to show that higher normal forms have been proposed D K N F is the ultimate normal form, but it remains an ideal rather than a practical normal form
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This concept is more properly known as "full functional dependence" Full functional dependence means that the LHS is minimal
Part Four
Relational Database Design
TABLE 73
StdSSN S1 S2
TJnnormalized University Database Table
StdCity SEATTLE BOTHELL StdClass JUN JUN OfferNo 01 02 03 02 OffTerm FALL FALL SPRING FALL OffYear 2006 2006 2007 2006 EnrGrade 35 33 31 34 CourseNo C1 C2 C3 C2 CrsDesc DB VB OO VB
First Normal Form
I N F prohibits nesting or repeating groups in tables A table not in I N F is unnormalized or nonnormalized In Table 73, the university table is unnormalized because the two rows contain repeating groups or nested tables To convert an unnormalized table into INF, you replace each value o f a repeating group with a row In a n e w row, y o u copy the nonrepeat ing columns You can see the conversion by comparing Table 73 with Table 71 (two rows with repeating groups versus four rows without repeating groups) Because most commercial D B M S s require I N F tables, y o u normally do not need to con vert tables into INF However, you often need to perform the reverse process ( I N F tables to unnormalized tables) for report generation A s discussed in 10, reports use nest ing to show relationships However, the underlying tables do not have nesting
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