外文翻譯---關(guān)系數(shù)據(jù)庫的結(jié)構(gòu)-其他專業(yè)-資料下載頁
2025-01-19 09:22本頁面
【導(dǎo)讀】關(guān)系模型是任何關(guān)系數(shù)據(jù)庫管理系統(tǒng)的基礎(chǔ)。一個(gè)關(guān)系模型有三個(gè)重要組成。一個(gè)關(guān)系數(shù)據(jù)庫使用關(guān)系或二維表來存儲(chǔ)支持某個(gè)事物所需的信息。個(gè)傳統(tǒng)的關(guān)系數(shù)據(jù)庫系統(tǒng)的基本組件并學(xué)習(xí)關(guān)系數(shù)據(jù)庫的設(shè)計(jì)。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)庫的強(qiáng)大功能。在關(guān)系數(shù)據(jù)庫中，一個(gè)表是一個(gè)用于保存相關(guān)信息的二維結(jié)構(gòu)。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)庫的幾項(xiàng)基本功能來管理人力資源部門。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。庫能夠規(guī)定列中的一個(gè)單元格是否為空。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。參照完整性的概念不只可以阻止無效的部。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。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。
【正文】 etomany. 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 pany 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 onetomany. Onetoone In a onetoone 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 mon to all employees, while the FULLTIME, PARTTIME, and CONTRACTOR tables hold information unique to fulltime employees, parttime employees, and contractors, respectively. These entities would be considered subtypes of an EMPLOYEE and maintain a onetoone relationship with the EMPLOYEE table. These relationships are not as mon as onetomany 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. Manytomany In a manytomany 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 manytomany 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 binations 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 plete, 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, it39。s the DBA39。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.
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