java aztec barcode library The Entity-Relationship Model in Software

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The Entity-Relationship Model
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But what if the discretionary budget is a sum that covers all departments managed by that employee In this case each Manages2 relationship that involves a given employee will have the same value in the dbudget eld In general such redundancy could be signi cant and could cause a variety of problems (We discuss redundancy and its attendant problems in 15) Another problem with this design is that it is misleading We can address these problems by associating dbudget with the appointment of the employee as manager of a group of departments In this approach, we model the appointment as an entity set, say Mgr Appt, and use a ternary relationship, say Manages3, to relate a manager, an appointment, and a department The details of an appointment (such as the discretionary budget) are not repeated for each department that is included in the appointment now, although there is still one Manages3 relationship instance per such department Further, note that each department has at most one manager, as before, because of the key constraint This approach is illustrated in Figure 217
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Entity Set versus Relationship
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253 Binary versus Ternary Relationships *
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Consider the ER diagram shown in Figure 218 It models a situation in which an employee can own several policies, each policy can be owned by several employees, and each dependent can be covered by several policies Suppose that we have the following additional requirements: A policy cannot be owned jointly by two or more employees Every policy must be owned by some employee
name ssn lot pname
2
Employees
Covers
Dependents
Policies
policyid
cost
Policies as an Entity Set
Dependents is a weak entity set, and each dependent entity is uniquely identi ed by taking pname in conjunction with the policyid of a policy entity (which, intuitively, covers the given dependent) The rst requirement suggests that we impose a key constraint on Policies with respect to Covers, but this constraint has the unintended side e ect that a policy can cover only one dependent The second requirement suggests that we impose a total participation constraint on Policies This solution is acceptable if each policy covers at least one dependent The third requirement forces us to introduce an identifying relationship that is binary (in our version of ER diagrams, although there are versions in which this is not the case) Even ignoring the third point above, the best way to model this situation is to use two binary relationships, as shown in Figure 219 This example really had two relationships involving Policies, and our attempt to use a single ternary relationship (Figure 218) was inappropriate There are situations, however, where a relationship inherently associates more than two entities We have seen such an example in Figure 24 and also Figures 215 and 217 As a good example of a ternary relationship, consider entity sets Parts, Suppliers, and Departments, and a relationship set Contracts (with descriptive attribute qty) that involves all of them A contract speci es that a supplier will supply (some quantity of) a part to a department This relationship cannot be adequately captured by a collection of binary relationships (without the use of aggregation) With binary relationships, we can denote that a supplier can supply certain parts, that a department needs some
The Entity-Relationship Model
name ssn lot pname age
Employees Purchaser Beneficiary
Dependents
Policies
policyid
cost
Policy Revisited
parts, or that a department deals with a certain supplier No combination of these relationships expresses the meaning of a contract adequately, for at least two reasons: The facts that supplier S can supply part P, that department D needs part P, and that D will buy from S do not necessarily imply that department D indeed buys part P from supplier S! We cannot represent the qty attribute of a contract cleanly
254 Aggregation versus Ternary Relationships *
As we noted in Section 245, the choice between using aggregation or a ternary relationship is mainly determined by the existence of a relationship that relates a relationship set to an entity set (or second relationship set) The choice may also be guided by certain integrity constraints that we want to express For example, consider the ER diagram shown in Figure 213 According to this diagram, a project can be sponsored by any number of departments, a department can sponsor one or more projects, and each sponsorship is monitored by one or more employees If we don t need to record the until attribute of Monitors, then we might reasonably use a ternary relationship, say, Sponsors2, as shown in Figure 220 Consider the constraint that each sponsorship (of a project by a department) be monitored by at most one employee We cannot express this constraint in terms of the Sponsors2 relationship set On the other hand, we can easily express the constraint by drawing an arrow from the aggregated relationship Sponsors to the relationship
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