计算机代写|数据库作业代写Database代考|MAPPING THE ENTITY DIAGRAM

Having illustrated the idea of the entity and the attribute, we now turn to a semi-physical realization of the concepts. We say semi-physical because we are not concerned with the actual physical file stored in memory; rather, we are concerned with placing data into relational tables we will visualize as a physical organization of data. Basically, a relational database is a database of two-dimensional tables called relations. The tables are composed of rows and columns. The rows are sometimes called tuples; the columns are attributes. In a relational database, all attributes (table columns) must be atomic, and keys must not be null. In addition, in relational databases, it is not necessary to know the actual physical location of the data on a magnetic device like a disk.

The process of converting an ER diagram into a database is called mapping. We concern ourselves only with the relational model; hence, as the chapters in this book develop, we will develop mapping rules to turn ER diagrams into relational databases.
Mapping rule 1-Mapping strong entities. Develop a new table (relation) for each strong entity and make the indicated key of the strong entity the primary key of the table. If more than one candidate key is indicated on the ER diagram, choose one for the primary key. Call this table, TABLEI.

Next, we must map the attributes into the strong entity’s table, TABLE1. Mapping rules are different for atomic attributes, composite attributes, and multivalued attributes. First, we present the mapping rule for mapping atomic attributes.

计算机代写|数据库作业代写Database代考|Beyond the First Entity Diagram

Now that we have devised a process for drawing, interpreting, and refining one primary entity, we need to move to more complex databases. To progress from here, we continue with our primary entity and look for other information associated with (related to) that entity.

The first technique employed in this chapter is methodical: We test our primary entity to see whether our attributes ought to be entities themselves. We then look for other pieces of information in our description, add them to:
(a) An existing entity and examine the existing entity-relationship (ER) diagram
(b) Create a new entity directly
After creating the new entities, we look to see what relationships exist between the two entities. A database is a collection of related data; hence, new entities may be added as long as there is a way to connect the new entity to existing ones. This chapter develops steps 3 through 5 of the ER design methodology presented in this book. Step 3 examines the attributes of the primary entity; step 4 discusses what to do if another entity is needed; and step 5 discusses developing the relationship between the two entities.

Although the concept of relationships is introduced in this chapter, we do not include any new mapping rules in this chapter since they can be better understood after the development of structural constraints on relationships, discussed in Chapter 6 . At the end of this chapter, we continue with the case started in Chapter 4 .

数据库代考

计算机代写|数据库作业代写Database代考|Beyond the First Entity Diagram

(a) 现有实体并检查现有实体关系 (ER) 图
(b) 直接创建新实体

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