講關(guān)系數(shù)據(jù)庫(kù)的結(jié)構(gòu)的計(jì)算機(jī)英語
計(jì)算機(jī)的英語是牽扯到很多專業(yè)的術(shù)語，所以學(xué)習(xí)起來有點(diǎn)困難，小編今天就給大家整理了有關(guān)于計(jì)算機(jī)的英語，有時(shí)間可以多多參考一下
計(jì)算機(jī)英語
Structure of the Relational database
The relational model is the basis for any relational database management system (RDBMS).A relational model has three core components: a collection of objects or relations, operators that act on the objects or relations, and data integrity methods. In other words, it has a place to store the data, a way to create and retrieve the data, and a way to make sure that the data is logically consistent.
A relational database uses relations, or two-dimensional tables, to store the information needed to support a business. Let's go over the basic components of a traditional relational database system and look at how a relational database is designed. Once you have a solid understanding of what rows, columns, tables, and relationships are, you'll be well on your way to leveraging the power of a relational database.
Tables, Row, and Columns
A table in a relational database, alternatively known as a relation, is a two-dimensional structure used to hold related information. A database consists of one or more related tables.
Note: Don't confuse a relation with relationships. A relation is essentially a table, and a relationship is a way to correlate, join, or associate two tables.
A row in a table is a collection or instance of one thing, such as one employee or one line item on an invoice. A column contains all the information of a single type, and the piece of data at the intersection of a row and a column, a field, is the smallest piece of information that can be retrieved with the database's query language. For example, a table with information about employees might have a column called LAST_NAME that contains all of the employees' last names. Data is retrieved from a table by filtering on both the row and the column.
Primary Keys, Datatypes, and Foreign Keys
The examples throughout this article will focus on the hypothetical work of Scott Smith, database developer and entrepreneur. He just started a new widget company and wants to implement a few of the basic business functions using the relational database to manage his Human Resources (HR) department.
Relation: A two-dimensional structure used to hold related information, also known as a table.
Note: Most of Scott's employees were hired away from one of his previous employers, some of whom have over 20 years of experience in the field. As a hiring incentive, Scott has agreed to keep the new employees' original hire date in the new database.
Row:A group of one or more data elements in a database table that describes a person, place, or thing.
Column:The component of a database table that contains all of the data of the same name and type across all rows.
You'll learn about database design in the following sections, but let's assume for the moment that the majority of the database design is completed and some tables need to be implemented. Scott creates the EMP table to hold the basic employee information, and it looks something like this:
Notice that some fields in the Commission (COMM) and Manager (MGR) columns do not contain a value; they are blank. A relational database can enforce the rule that fields in a column may or may not be empty. In this case, it makes sense for an employee who is not in the Sales department to have a blank Commission field. It also makes sense for the president of the company to have a blank Manager field, since that employee doesn't report to anyone.
Field:The smallest piece of information that can be retrieved by the database query language. A field is found at the intersection of a row and a column in a database table.
On the other hand, none of the fields in the Employee Number (EMPNO) column are blank. The company always wants to assign an employee number to an employee, and that number must be different for each employee. One of the features of a relational database is that it can ensure that a value is entered into this column and that it is unique. The EMPNO column, in this case, is the primary key of the table.
Primary Key:A column (or columns) in a table that makes the row in the table distinguishable from every other row in the same table.
Notice the different datatypes that are stored in the EMP table: numeric values, character or alphabetic values, and date values.
As you might suspect, the DEPTNO column contains the department number for the employee. But how do you know what department name is associated with what number? Scott created the DEPT table to hold the descriptions for the department codes in the EMP table.
The DEPTNO column in the EMP table contains the same values as the DEPTNO column in the DEPT table. In this case, the DEPTNO column in the EMP table is considered a foreign key to the same column in the DEPT table.
A foreign key enforces the concept of referential integrity in a relational database. The concept of referential integrity not only prevents an invalid department number from being inserted into the EMP table, but it also prevents a row in the DEPT table from being deleted if there are employees still assigned to that department.
Foreign Key:A column (or columns) in a table that draws its values from a primary or unique key column in another table. A foreign key assists in ensuring the data integrity of a table. Referential IntegrityA method employed by a relational database system that enforces one-to-many relationships between tables.
Data Modeling
Before Scott created the actual tables in the database, he went through a design process known as data modeling. In this process, the developer conceptualizes and documents all the tables for the database. One of the common methods for modeling a database is called ERA, which stands for entities, relationships, and attributes. The database designer uses an application that can maintain entities, their attributes, and their relationships. In general, an entity corresponds to a table in the database, and the attributes of the entity correspond to columns of the table.
Data Modeling:A process of defining the entities, attributes, and relationships between the entities in preparation for creating the physical database.
The data-modeling process involves defining the entities, defining the relationships between those entities, and then defining the attributes for each of the entities. Once a cycle is complete, it is repeated as many times as necessary to ensure that the designer is capturing what is important enough to go into the database. Let's take a closer look at each step in the data-modeling process.
Defining the Entities
First, the designer identifies all of the entities within the scope of the database application.The entities are the persons, places, or things that are important to the organization and need to be tracked in the database. Entities will most likely translate neatly to database tables. For example, for the first version of Scott's widget company database, he identifies four entities: employees, departments, salary grades, and bonuses. These will become the EMP, DEPT, SALGRADE, and BONUS tables.
Defining the Relationships Between Entities
Once the entities are defined, the designer can proceed with defining how each of the entities is related. Often, the designer will pair each entity with every other entity and ask, "Is there a relationship between these two entities?" Some relationships are obvious; some are not.
In the widget company database, there is most likely a relationship between EMP and DEPT, but depending on the business rules, it is unlikely that the DEPT and SALGRADE entities are related. If the business rules were to restrict certain salary grades to certain departments, there would most likely be a new entity that defines the relationship between salary grades and departments. This entity would be known as an associative or intersection table and would contain the valid combinations of salary grades and departments.
Associative Table:A database table that stores the valid combinations of rows from two other tables and usually enforces a business rule. An associative table resolves a many-to-many relationship.
In general, there are three types of relationships in a relational database:
One-to-many The most common type of relationship is one-to-many. This means that for each occurrence in a given entity, the parent entity, there may be one or more occurrences in a second entity, the child entity, to which it is related. For example, in the widget company database, the DEPT entity is a parent entity, and for each department, there could be one or more employees associated with that department. The relationship between DEPT and EMP is one-to-many.
One-to-one In a one-to-one relationship, a row in a table is related to only one or none of the rows in a second table. This relationship type is often used for subtyping. For example, an EMPLOYEE table may hold the information common to all employees, while the FULLTIME, PARTTIME, and CONTRACTOR tables hold information unique to full-time employees, part-time employees, and contractors, respectively. These entities would be considered subtypes of an EMPLOYEE and maintain a one-to-one relationship with the EMPLOYEE table. These relationships are not as common as one-to-many relationships, because if one entity has an occurrence for a corresponding row in another entity, in most cases, the attributes from both entities should be in a single entity.
Many-to-many In a many-to-many relationship, one row of a table may be related to many rows of another table, and vice versa. Usually, when this relationship is implemented in the database, a third entity is defined as an intersection table to contain the associations between the two entities in the relationship. For example, in a database used for school class enrollment, the STUDENT table has a many-to-many relationship with the CLASS table—one student may take one or more classes, and a given class may have one or more students. The intersection table STUDENT_CLASS would contain the combinations of STUDENT and CLASS to track which students are in which classes.
Assigning Attributes to Entities
Once the designer has defined the entity relationships, the next step is to assign the attributes to each entity. This is physically implemented using columns, as shown here for the SALGRADE table as derived from the salary grade entity.
After the entities, relationships, and attributes have been defined, the designer may iterate the data modeling many more times. When reviewing relationships, new entities may be discovered. For example, when discussing the widget inventory table and its relationship to a customer order, the need for a shipping restrictions table may arise.
Once the design process is complete, the physical database tables may be created. Logical database design sessions should not involve physical implementation issues, but once the design has gone through an iteration or two, it's the DBA's job to bring the designers "down to earth." As a result, the design may need to be revisited to balance the ideal database implementation versus the realities of budgets and schedules.
關(guān)系數(shù)據(jù)庫(kù)的結(jié)構(gòu)
關(guān)系模型是任何關(guān)系數(shù)據(jù)庫(kù)管理系統(tǒng)(RDBMS)的基礎(chǔ)。一個(gè)關(guān)系模型有二個(gè)核心組件：對(duì)象或關(guān)系的集合，作用于對(duì)象或關(guān)系上的操作，以及數(shù)據(jù)完整性規(guī)則。換句話說，關(guān)系數(shù)據(jù)庫(kù)有一個(gè)存儲(chǔ)數(shù)據(jù)的地方，一種創(chuàng)建和檢索數(shù)據(jù)的方法，以及一種確認(rèn)數(shù)據(jù)的邏輯一致性的方法。
一個(gè)關(guān)系數(shù)據(jù)庫(kù)使用關(guān)系或二維表來存儲(chǔ)支持某個(gè)事物所需的信息。讓我們了解一下一個(gè)傳統(tǒng)的關(guān)系數(shù)據(jù)庫(kù)系統(tǒng)的基本組件并目學(xué)習(xí)如何設(shè)計(jì)一個(gè)關(guān)系數(shù)據(jù)庫(kù)。一旦你對(duì)于行、列、表和關(guān)聯(lián)是什么有了深刻理解，你就能夠充分發(fā)揮關(guān)系數(shù)據(jù)庫(kù)的強(qiáng)大功能。
表，行和列
在關(guān)系數(shù)據(jù)庫(kù)中，一個(gè)表(或者說一個(gè)關(guān)系)是一個(gè)用于保存相關(guān)信息的二維結(jié)構(gòu)。一個(gè)數(shù)據(jù)庫(kù)由一個(gè)或者多個(gè)相關(guān)聯(lián)的表組成。
注意：不要混淆了關(guān)系和關(guān)聯(lián)。一個(gè)關(guān)系實(shí)際上是一個(gè)表，而一個(gè)關(guān)聯(lián)指的是一種連接、結(jié)合或聯(lián)合兩個(gè)表的方式。
表中的一行是一種事物的集合或?qū)嵗热缫粋€(gè)員工或發(fā)票上的一項(xiàng)。表中的一列包含了一類信息;而且行列交叉點(diǎn)上的數(shù)據(jù)，字段，即是能夠用數(shù)據(jù)庫(kù)查詢語言檢索到的最小片信息。舉個(gè)例子來說，一個(gè)員工信息表可能有一個(gè)“名字”列，列中就包含所有員工的名字。數(shù)據(jù)是通過對(duì)行、列進(jìn)行過濾而從表中檢索出來的。
主碼、數(shù)據(jù)類型和外碼
本篇文章均以假設(shè)的斯科特·史密斯的工廠為例，他是數(shù)據(jù)庫(kù)的建立者和企業(yè)的主辦人。他剛開辦了一個(gè)飾品公司并目想要使用關(guān)系數(shù)據(jù)庫(kù)的幾項(xiàng)基本功能來管理人力資源部門。
關(guān)系：用來保存相關(guān)信息的一個(gè)二維結(jié)構(gòu)(也就是表)。
注意：大多數(shù)斯科特的雇員都是雇自過去的從業(yè)者，他們中有些人在這個(gè)領(lǐng)域己經(jīng)有20年的經(jīng)驗(yàn)了。出于雇用的目的，斯科特同意在新數(shù)據(jù)庫(kù)中維持新進(jìn)員工最初的雇傭日期。
行：在一個(gè)數(shù)據(jù)庫(kù)表中的一組單數(shù)據(jù)或多數(shù)據(jù)元素，用于描述一個(gè)人、地方或事物。
列：列是數(shù)據(jù)庫(kù)表的組件，它包含所有行中同名和同類型的所有數(shù)據(jù)。
你會(huì)在下面章節(jié)學(xué)到如何設(shè)計(jì)數(shù)據(jù)庫(kù)，現(xiàn)在讓我們假設(shè)數(shù)據(jù)庫(kù)大部分己經(jīng)設(shè)計(jì)完成并且有一些表需要被執(zhí)行。斯科特創(chuàng)建了EMP表來保存基本的員工信息，就像這樣：
你可能注意到傭金列和管理人列中有一些單元格中沒有值;它們是空值。一個(gè)關(guān)系數(shù)據(jù)庫(kù)能夠規(guī)定列中的一個(gè)單元格是否為空。如此，可以明確那些非銷售部的員工傭金單元為空。同樣也明確了公司董事長(zhǎng)的管理人單元為空，因?yàn)檫@個(gè)員工不需要向任何人匯報(bào)工作。
單元格：是數(shù)據(jù)庫(kù)查詢語言所能夠檢索到的最小片信息。一個(gè)單元格就是一個(gè)數(shù)據(jù)庫(kù)表的行和列交叉形成的。
另一方面，沒有哪個(gè)員工的員工編號(hào)單元為空。公司總是希望為每個(gè)員工分配一個(gè)員工號(hào)，并目這個(gè)號(hào)碼必須是每個(gè)員工都不同的。關(guān)系數(shù)據(jù)庫(kù)的一個(gè)特性能夠確定某列的鍵入值必須為單值。如此，員工編號(hào)列便是這個(gè)表的主碼。
主碼：主碼即是表中的一列(或多列)，使每一行能夠區(qū)別于同表中的其他行。
留意一下EMP表中存儲(chǔ)的不同數(shù)據(jù)類型：數(shù)值型，字符型或字母型，以及日期型。
如你所想，部門成員列保存的是員工所在部門的編號(hào)。但是你如何知道哪個(gè)部門名稱對(duì)應(yīng)哪個(gè)部門編號(hào)呢?斯科特建立了DEPT表來具體描述EMP表中提到的部門編號(hào)的情況。
EMP表中的部門編號(hào)列同DEPT表中的部門編號(hào)列有著相同的值。既然如此，EMP表中的部門編號(hào)列便被看作是與DEPT表中相同列對(duì)應(yīng)的外碼。
外碼加強(qiáng)了關(guān)系數(shù)據(jù)庫(kù)中參考完整性的概念。參考完整性的概念不只可以阻止無效的部門編號(hào)被插入EMP表中，而且在某部門仍有員工的情況下，可以防止DEPT表中該部門的信息被刪除。
外碼：表中的一列(或多列)，它的值來自于其他表的主碼列或單值列。一個(gè)外碼有助于確定表中數(shù)據(jù)的完整性。
參考完整性：是關(guān)系數(shù)據(jù)庫(kù)用來加強(qiáng)表間一對(duì)多關(guān)聯(lián)的一種方式。
數(shù)據(jù)建模
在斯科特于數(shù)據(jù)庫(kù)中創(chuàng)建真實(shí)表之前，他要經(jīng)過一個(gè)稱作數(shù)據(jù)建模的過程。在這個(gè)過程中，數(shù)據(jù)庫(kù)創(chuàng)建者定義和填寫數(shù)據(jù)庫(kù)中所有表。有一種為數(shù)據(jù)庫(kù)建模的方式叫作ERA,它可以表示出實(shí)體、實(shí)體間的關(guān)聯(lián)和實(shí)體的屬性。數(shù)據(jù)庫(kù)設(shè)計(jì)者使用一個(gè)能夠支持實(shí)體、實(shí)體屬性和實(shí)體間關(guān)聯(lián)的應(yīng)用程序。通常，一個(gè)實(shí)體對(duì)應(yīng)數(shù)據(jù)庫(kù)中的一個(gè)表，而實(shí)體的屬性對(duì)應(yīng)于表中的列。
數(shù)據(jù)建模：一個(gè)定義實(shí)體、實(shí)體屬性和實(shí)體間關(guān)聯(lián)的過程，從而為建立物理數(shù)據(jù)庫(kù)做準(zhǔn)備。
數(shù)據(jù)建模過程包括定義實(shí)體、定義實(shí)體間關(guān)聯(lián)以及定義每個(gè)實(shí)體的屬性的過程。一旦一個(gè)周期完成，就需要不斷重復(fù)直到設(shè)計(jì)者抓住了重點(diǎn)，足以開始建立數(shù)據(jù)庫(kù)。讓我們進(jìn)一步了解為數(shù)據(jù)庫(kù)建模過程的步驟。
定義實(shí)體
首先，設(shè)計(jì)者確定數(shù)據(jù)庫(kù)應(yīng)用程序范圍內(nèi)的所有實(shí)體。實(shí)體是人、地方或事物，它們對(duì)于整個(gè)團(tuán)體是重要的且需要被記錄在數(shù)據(jù)庫(kù)中。實(shí)體將被巧妙的轉(zhuǎn)化為數(shù)據(jù)表。比如，在第一版斯科特飾品公司數(shù)據(jù)庫(kù)中，他定義了四個(gè)實(shí)體：?jiǎn)T工、部門、工資水平和獎(jiǎng)金。它們將稱為EMP(員工)表，DEPT(部門)表，SALGRADE(工資水平)表和BONUS(獎(jiǎng)金)表。
定義實(shí)體間的關(guān)聯(lián)
一旦定義了實(shí)體，設(shè)計(jì)者就能夠繼續(xù)定義每個(gè)實(shí)體間是如何關(guān)聯(lián)的。通常，設(shè)計(jì)者通常將每個(gè)實(shí)體同其他實(shí)體配對(duì)，并目考慮：“兩者之間是否存在關(guān)聯(lián)呢?”實(shí)體間的某些關(guān)聯(lián)是明顯的，某些不是。
在飾品公司數(shù)據(jù)庫(kù)中，員工實(shí)體和部門實(shí)體間極可能存在關(guān)聯(lián)，而依據(jù)事物間的關(guān)系原則，部門實(shí)體跟工資水平實(shí)體間似乎就沒有關(guān)聯(lián)了。如果事物間的關(guān)系原則是用來約束某個(gè)部門的工資水平的，就可能需要一個(gè)新的實(shí)體來說明工資水平和部門之間的關(guān)聯(lián)。這個(gè)實(shí)體被稱作關(guān)系表或交表，其中包含工資水平和部門之間的有效聯(lián)合。
關(guān)系表：是一個(gè)數(shù)據(jù)庫(kù)表，其中保存著另外兩個(gè)表的行(記錄)間的有效結(jié)合，并且通常強(qiáng)調(diào)了事物間的關(guān)系原則。關(guān)聯(lián)表處理的是一個(gè)多對(duì)多關(guān)聯(lián)。
通常，關(guān)系數(shù)據(jù)庫(kù)間有二種關(guān)聯(lián)方式：
·一對(duì)多關(guān)聯(lián)：最常見的關(guān)聯(lián)是一對(duì)多關(guān)聯(lián)。意思是對(duì)于每個(gè)給出的現(xiàn)有實(shí)體(即父實(shí)體)都有一個(gè)或多個(gè)現(xiàn)有的另一個(gè)實(shí)體(即子實(shí)體)與之相關(guān)聯(lián)。舉個(gè)例子來說，在飾品公司數(shù)據(jù)庫(kù)中，部門實(shí)體是一個(gè)父實(shí)體，而每個(gè)部門中，都有一個(gè)或多個(gè)員工屬于該部門。這樣，部門實(shí)體和員工實(shí)體間的關(guān)聯(lián)就是一對(duì)多關(guān)聯(lián)。
·一對(duì)一關(guān)聯(lián)：在一個(gè)一對(duì)一關(guān)聯(lián)中，表中的一行只關(guān)聯(lián)另一個(gè)表中的一行甚至0行。這種關(guān)聯(lián)類型通常用于子類型數(shù)據(jù)中。例如，一個(gè)員工表可能保存了所有員工的信息，而全職表、兼職表和承包人表則分別保存全職員工、兼職員工和承包人的信息。這些實(shí)體被認(rèn)為是員工表的子表，并目同員工表維持一對(duì)一關(guān)聯(lián)。這種關(guān)系不像一對(duì)多關(guān)聯(lián)那么常見，因?yàn)槿绻粋€(gè)實(shí)體與另一個(gè)實(shí)體總有對(duì)應(yīng)行，在大多數(shù)情況下，兩個(gè)實(shí)體中的屬性只在一個(gè)實(shí)體內(nèi)出現(xiàn)就可以了。
·多對(duì)多關(guān)聯(lián)：在多對(duì)多關(guān)聯(lián)中，表的一行可能對(duì)應(yīng)另一個(gè)表的許多行，反之亦然。通常，當(dāng)這些關(guān)聯(lián)在數(shù)據(jù)庫(kù)中被執(zhí)行時(shí)，往往再定義第三個(gè)實(shí)體用來保存前兩個(gè)實(shí)體間的所有關(guān)聯(lián)。例如，在一個(gè)學(xué)籍注冊(cè)數(shù)據(jù)庫(kù)中，學(xué)生表與班級(jí)表之間有一個(gè)多對(duì)多關(guān)聯(lián)——一個(gè)學(xué)生可能聽一門或多門課程，并目一個(gè)班級(jí)也可能有一個(gè)或多個(gè)學(xué)生。而學(xué)生_班級(jí)關(guān)系表中就包含了學(xué)生和班級(jí)之間的關(guān)系，以表明哪個(gè)學(xué)生在哪個(gè)班。
指定實(shí)體屬性
一旦設(shè)計(jì)者定義了實(shí)體間關(guān)聯(lián)，下一步就是去指定每個(gè)實(shí)體的屬性。這是實(shí)現(xiàn)列的使用，如右圖所示由工資水平實(shí)體所建立的工資水平表。
重復(fù)步驟：我們?nèi)匀辉谠?/p>
在定義了實(shí)體、關(guān)聯(lián)以及屬性之后，設(shè)計(jì)者往往要多重復(fù)幾次數(shù)據(jù)建模過程。當(dāng)我們?cè)诨仡欔P(guān)聯(lián)時(shí)，就會(huì)發(fā)現(xiàn)需要建立新的實(shí)體。比如，當(dāng)討論飾品庫(kù)存表和與它相關(guān)的客戶訂單時(shí)，就會(huì)發(fā)現(xiàn)需要制定一個(gè)送貨約束表。
一旦設(shè)計(jì)過程完成，下面將要建立實(shí)際的數(shù)據(jù)庫(kù)表。邏輯數(shù)據(jù)庫(kù)的設(shè)計(jì)過程不會(huì)牽涉實(shí)際執(zhí)行中的問題。然而，一旦設(shè)計(jì)進(jìn)入到實(shí)際的運(yùn)作，數(shù)據(jù)庫(kù)管理員就會(huì)很快讓設(shè)計(jì)者從理想回到現(xiàn)實(shí)中來。結(jié)果，設(shè)計(jì)就可能需要再次構(gòu)想以求得理想的數(shù)據(jù)庫(kù)存運(yùn)行與預(yù)算和進(jìn)度之間的平衡。
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