外文翻譯---關(guān)系數(shù)據(jù)庫的結(jié)構(gòu)-數(shù)據(jù)庫-文庫吧資料
2025-05-22 10:37本頁面
【正文】 tity, 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.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 take a closer look at each step in the datamodeling process.Defining the EntitiesFirst, the designer identifies all of the entities within the scope of the database 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 Scott39。 they are blank. A rela
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