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2: Exploring Relational Database Components
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recognize this data as the Order Detail table from Northwind s relational model So why isn t Order Detail just shown as an entity The answer is simple: It doesn t fit the definition of an entity We are not collecting data about the line items on the order; instead, the line items on the order are merely more data about the order Many-to-many relationships are quite common, and most of them will have intersection data The bad news is that the relational model does not directly support many-to-many relationships There is no problem with having many-to-many relationships in a conceptual design, because such a design is independent of any particular technology However, if the database is going to be relational, some changes have to be made as you map the conceptual model to the corresponding logical model The solution is to map the intersection data to a separate table (an intersection table) and the many-tomany relationship to two, one-to-many relationships, with the intersection table in the middle and on the many side of both relationships Figure 1-7 shows this outcome, with the Order Detail table holding the intersection data and participating in two one-tomany relationships that replace the original many-to-many relationship The process for recognizing and dealing with the many-to-many problem is covered in detail in 6
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Recursive Relationships
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So far, you ve learned about relationships between instances of different entities However, relationships can exist between entity instances of the same type These are called recursive relationships Any one of the relationship types already presented (one-to-one, one-to-many, or many-to-many) can be a recursive relationship Figure 2-4 and the following list show examples of each:
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One-to-one If we were to track which employees were married to other employees, we would expect each to be married to either zero or one other employee at any one point in time
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Employee Employee ID Last Name First Name Job Title Manager Employee ID (FK) One-to-many: An employee can manage other employees
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Employee Employee ID Last Name First Name Job Title Spouse Employee ID (FK) One-to-one: Each employee can be married to another employee or not
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Many-to-many: Each part can contain other parts; each part can be a component of many other parts
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Figure 2-4
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Recursive relationship examples
Databases: A Beginner s Guide
One-to-many It is common to track the employment food chain of who reports to whom In most organizations, people have only one supervisor or manager Therefore, we normally expect to see each employee reporting to zero or one other employee, and employees who are managers or supervisors to have one or more direct reports Many-to-many In manufacturing, a common relationship has to do with parts that make up a finished product If you think about the CD-ROM drive in a personal computer, for example, you can imagine that it comprises multiple parts, and yet, the entire assembly shows as only one item on the parts list for your computer So any part can be made of many other parts, and at the same time any part can be a component of many other parts
Business Rules
A business rule is a policy, procedure, or standard that an organization has adopted Business rules are very important in database design because they dictate controls that must be placed upon the data In Figure 2-1, you can see a business rule that states that orders will be accepted only from customers who do not have an overdue balance Most business rules can be enforced through manual procedures that employees are directed to follow or logic placed in the application programs However, each of these can be circumvented employees can forget or can choose not to follow a manual procedure, and databases can be updated directly by authorized people, bypassing the controls included in the application programs The database can serve nicely as the last line of defense Business rules can be implemented in the database as constraints, which are formally defined rules that restrict the data values in the database in some way More information on constraints can be found in the Constraints section later in this chapter Note that business rules are not normally shown on a conceptual data model diagram; the one shown in Figure 2-1 is merely for illustration It is far more common to include them in a text document that accompanies the diagram
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