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Overview of Relational Database Design
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After converting an E R D to relational tables, your work is not yet finished You need to analyze the tables for redundancies that can make the tables difficult to use This section describes why redundancies can make a table difficult to use and presents an important kind o f constraint to analyze redundancies
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A g o o d database design ensures that users can change the contents o f a database without unexpected side effects For example, with a university database, a user should be able to insert a n e w course without having to simultaneously insert a n e w offering o f the course and a new student enrolled in the course Likewise, when a student is deleted from the database due to graduation, course data should not be inadvertently lost These problems
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modification anomaly an unexpected side effect that occurs when changing the data in a table with excessive redundancies
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are examples o f modification anomalies, unexpected side effects that occur w h e n changing the contents o f a table with excessive redundancies A g o o d database design avoids modi fication anomalies by eliminating excessive redundancies To understand more precisely the impact o f modification anomalies, let us consider a poorly designed database Imagine that a university database consists o f the single table shown in Table 71 Such a poor design makes it easy to identify anomalies The following list describes s o m e o f the problems with this design
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This table has insertion anomalies A n insertion anomaly occurs w h e n extra data beyond the desired data must be added to the database For example, to insert a course, it is necessary to know a student and an offering because the combination o f StdSSN and OfferNo is the primary key Remember that a row cannot exist with null values for part o f its primary key
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This table has update anomalies A n update anomaly occurs w h e n it is necessary to change multiple rows to modify only a single fact For example, if w e change the StdClass o f student S I , two rows must be changed If SI was enrolled in 10 classes, 10 rows must be changed
This table has deletion anomalies A deletion anomaly occurs whenever deleting a row inadvertently causes other data to be deleted For example, if we delete the enrollment o f S2 in 0 3 (third row), w e lose the information about offering 0 3 and course C3 To deal with these anomalies, users may circumvent them (such as using a default
primary key to insert a new course) or database programmers may write code to prevent inadvertent loss o f data A better solution is to modify the table design to remove the redundancies that cause the anomalies
TABLE 71
StdSSN S1 S1 S2 S2
Sample Data for the Big University Database Table
StdCity SEATTLE SEATTLE BOTHELL BOTHELL StdClass JUN JUN 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
This single-table design is not as extreme as it may seem Users without proper database training often design a database using a single table
7 Normalization of Relational Tables 221
FIGURE 71 Classification of Database Constraints
Constraint
Value-based Value-neutral
Functional Dependencies
Functional dependencies are important tools w h e n analyzing a table for excessive redun dancies A functional dependency is a constraint about the database contents Constraints can be characterized as value-based versus value-neutral (Figure 71) A value-based constraint involves a comparison o f a column to a constant using a comparison operator such as <, =, or > For example, age > 21 is an important value-based constraint in a data base used to restrict sales o f alcohol to minors A value-neutral constraint involves a c o m parison o f columns For example, a value-neutral constraint is that retirement age should be greater than current age in a database for retirement planning Primary key (PK) and foreign key (FK) constraints are important kinds o f value-neutral constraints A primary key can take any value as long as it does not match the primary k e y value in an existing row A foreign key constraint requires that the value o f a column in o n e table match the value o f a primary key in another table A functional dependency is another important kind o f value-neutral constraint A
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