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# 计算机代写|数据库代考Database代考|CMSC724 The Entity–Relationship Model

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## 计算机代写|数据库代考Database代考|The Entity–Relationship Model

The popular answer to the challenge of database representation is the entity-relationship diagram (ERD or E-R diagram). Figure $3.2$ shows the symbols used in an ERD. As the figure conveys, there are two widely used conventions for constructing ERDs: the Crow’s Foot notation and the Chen notation. Other notations – for example, the Integration Definition for Information Modeling (IDEF1X) – have been proposed by various software engineering firms, but they will not be discussed here. Figure $3.3$ provides an illustrative ERD based on the Crow’s Foot notation. In the diagram, the convention to show attributes of each entity has been relaxed, thus avoiding clutter. Note also that relationships are labeled as verbs so that in linking one entity to another, one can read an entity-verb-entity formulation. If the verb is on the right or above the relationship line, the convention is to read from top to bottom or left to right. If the verb is on the left or below the relationship line, the convention is to read from bottom to top or right to left.

As you observe Figure 3.3, you will notice that there are 17 entities and 18 relationships connecting them. Here are the entities: Customer, PurchaseInvoice, Supplier, PurchaseOrder, Machine, Warehouse, InventoryItem, Machine, Project, Location, Department, Employee, EmployeePe rsonalInfo, EmployeeEmploymentHistory, EmployeeAcademicLog, EmployeePublications, and EmployeeExtraCurricular. Also note that each relationship is given a name. How you name the relationship depends on your diagramming convention. The relationship-name connecting any two entities is typically a verb. If you are using the Chen notation, the relationship-name goes inside the diamond (review Figure 3.2). If you are using the Crow’s Foot notation, the naming abides by the following rule: If you read from left to right, the name goes above the relationship line; if you proceed from right to left, the name goes underneath the relationship line. If you read from top to bottom, the name goes to the right of the relationship line; if you proceed from bottom to top, the name goes to the left of the relationship line.

## 计算机代写|数据库代考Database代考|The Object–Relationship Model

As you are aware, or will soon learn (from your software engineering course), there are, broadly speaking, two alternate paradigms for software construction: the functional approach (which is the traditional approach) and the object-oriented (OO) approach. In an object-oriented environment, the comparative methodology for the $\mathrm{E}-\mathrm{R}$ diagram is the object-relationship diagram (ORD or O-R diagram). The concept of an ORD is similar to that of an ERD, and the diagrams are also similar, but there are a few exceptions:

• In the OO paradigm, the object type replaces the entity (type) of the relational model. Like the entity, an object type is a concept or thing about which data is stored. Additionally, the object type defines a set of operations, which will be applicable to all objects (instances) of that type.
• The symbol used to denote an object type is similar to an entity symbol, except that it has two extended areas – one for the attributes of the object type and the other for its operations.
• The preferred diagramming convention is the UML (Unified Modeling Language) notation.
• Depending on the OO development tool, there might be additional notations regarding the cardinality (more precisely, multiplicity) of the relationships represented.

Mastery of the OO approach to database systems is contingent on a good grasp of the relational model. Moreover, a full treatment is beyond the scope of this course. For these reasons, further discussion is deferred until Chapter 5 (Sections $5.6$ and 5.7) and Chapter 18. You are no doubt familiar with using UML diagrams in your $\mathrm{OO}$ programming courses. However, as you will learn in Chapter 5, and more comprehensively from your software engineering course, there is a lot more to UML than just programming. For a quick review of the fundamentals, please see references Lee (2002) and Schach (2011).

## 计算机代写|数据库代考Database代考|The Object–Relationship Model

• 在 OO 范式中，对象类型取代了关系模型的实体（类型）。与实体一样，对象类型是关于存储哪些数据的概念或事物。此外，对象类型定义了一组操作，这些操作将适用于该类型的所有对象（实例）。
• 用于表示对象类型的符号类似于实体符号，只是它有两个扩展区域——一个用于对象类型的属性，另一个用于对象类型的操作。
• 首选的图表约定是 UML（统一建模语言）符号。
• 根据 OO 开发工具的不同，可能会有关于所表示关系的基数（更准确地说，多重性）的附加符号。

## MATLAB代写

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