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计算机代写|数据库作业代写Database代考|The Multivalued Attribute

Another type of non-simple attribute to be managed is called a multivalued attribute. The multivalued attribute, as the name implies, may take on more than one value for a given occurrence of an entity. For example, the attribute school could easily be multivalued if a person attends (or has attended, depending on the context of the database) more than one school. As a counter example, most people go by only one name; hence, the name is not multivalued.

The multivalued attribute called school is depicted in Figure $4.5 \mathrm{~A}$ (Chen-like model) as a double oval; this illustrates the situation for which a database will store data about students who may have attended more than one school. Although we have chosen to illustrate school as a multivalued attribute, we do not mean to imply this will always be the case in all databases. In fact, the attribute school may well be single valued in some databases. The idea of school may mean the current (or just previous) school as opposed to all schools attended. If the subjects about whom we are storing data can attend only one school at a time (and that is what we want to depict), then the attribute school may well be a single valued attribute.

Again, the test of single versus multivalued will depend entirely on the circumstances the database designer encounters-the desire of the user of the to-be-built database. It is recommended if the sense of the database is the attribute school means “current school,” then the attribute should be called “current school” and illustrated as a single-valued attribute. We show a multivalued attribute in Figure 4.5A. This diagram implies multiple schools may be recorded for each student.

计算机代写|数据库作业代写Database代考|The Derived Attribute

A database is used to store data for retrieval. An attribute used to find a particular entity occurrence is called a key. As we model our database with the ER models, we may find some attributes naturally seem to be keys.

If an attribute nay be thought of as a unique identifier for an entity, it is called a candidate key. When a candidate key is chosen to be the unique identifier, it becomes the primary key for the entity.

As an example of keys, suppose we add an attribute called student_ number to our STUDENT entity example. We might well consider a student_number to be a unique identifier for the entity-a candidate key because of uniqueness. A name is often unique, but not necessarily so. Members of the same class often share last names. Address may or may not be a unique identifier and hence is not a likely candidate key. Siblings who take classes together could easily have the same address. Schools often choose to assign a unique student number to each student to be able to find student records. The idea of a key is to provide a unique way to find an entity instance (a particular record).

Some schools also choose to record a Social Security number (SSN) as an attribute. An SSN is also unique and hence a candidate key along with student_number. If both $S S N$ and student_number were recorded, then the designer would have to choose which candidate key would be the primary key. In our case, we choose not to record an SSN. The STUDENT entity with the unique identifier student_number, added as a key, is depicted in Figure 4.6.

In the Chen-like ER model unique identifiers (candidate keys), are usually underlined (as shown in Figure 4.6). A unique identifier may be an attribute or a combination of attributes. It is not necessary to choose which candidate key will be the primary key at this point, but one could do so. When there is only one candidate key, we will generally speak of it as the primary key simply because it is obvious the primary key is a candidate key. In Figure 4.6, we also depict a short form of the ER diagram (at the bottom of the figure) with composite attributes and multivalued attributes as well as primary keys. The composite attributes are listed with its component parts, and the multivalued attributes are enclosed in parentheses in the abbreviated form.

计算机代写|数据库作业代写Database代考|CMU15-445

数据库代考

计算机代写|数据库作业代写Database代考|The Multivalued Attribute

另一种要管理的非简单属性称为多值属性。顾名思义,多值属性对于给定的实体出现可能具有多个值。例如,如果一个人就读(或已经就读,取决于数据库的上下文)多于一所学校,则学校属性很容易成为多值。作为反例,大多数人只有一个名字。因此,名称不是多值的。

称为学校的多值属性如图所示4.5 一个(陈样模型)为双椭圆形;这说明了数据库将存储有关可能就读于不止一所学校的学生的数据的情况。虽然我们选择将学校作为一个多值属性来说明,但我们并不意味着这在所有数据库中都是如此。事实上,属性 school 在某些数据库中很可能是单值的。学校的概念可能意味着当前(或之前)的学校,而不是所有就读的学校。如果我们正在存储数据的对象一次只能上一所学校(这就是我们想要描述的),那么属性学校很可能是一个单值属性。

同样,单值与多值的测试将完全取决于数据库设计者遇到的情况——待建数据库用户的需求。建议如果数据库意义上的属性school表示“当前学校”,则该属性应称为“当前学校”,并说明为单值属性。我们在图 4.5A 中展示了一个多值属性。此图意味着可以为每个学生记录多个学校。

计算机代写|数据库作业代写Database代考|The Derived Attribute

数据库用于存储数据以供检索。用于查找特定实体出现的属性称为键。当我们使用 ER 模型对数据库建模时,我们可能会自然而然地发现一些属性似乎是键。

如果一个属性可以被认为是一个实体的唯一标识符,它就被称为候选键。当候选键被选为唯一标识符时,它就成为实体的主键。

作为键的示例,假设我们将一个名为 student_number 的属性添加到我们的 STUDENT 实体示例中。我们可能会将 student_number 视为实体的唯一标识符 – 由于唯一性而成为候选键。名称通常是唯一的,但不一定如此。同一班级的成员经常共享姓氏。地址可能是也可能不是唯一标识符,因此不是可能的候选键。一起上课的兄弟姐妹很容易拥有相同的地址。学校通常选择为每个学生分配一个唯一的学生编号,以便能够找到学生记录。键的想法是提供一种独特的方法来查找实体实例(特定记录)。

一些学校还选择将社会安全号码 (SSN) 记录为一个属性。SSN 也是唯一的,因此是与 student_number 一起的候选键。如果两者小号小号否和 student_number 被记录下来,那么设计者就必须选择哪个候选键作为主键。在我们的例子中,我们选择不记录 SSN。具有唯一标识符 student_number 的 STUDENT 实体作为键添加,如图 4.6 所示。

在 Chen-like ER 模型中,唯一标识符(候选键)通常带有下划线(如图 4.6 所示)。唯一标识符可以是一个属性或多个属性的组合。此时没有必要选择哪个候选键为主键,但可以这样做。当只有一个候选键时,我们通常将其称为主键,因为很明显主键是一个候选键。在图 4.6 中,我们还描述了带有复合属性和多值属性以及主键的 ER 图的简短形式(在图的底部)。复合属性与其组成部分一起列出,多值属性以缩写形式括在括号中。

计算机代写|数据库作业代写Database代考

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