數(shù)據(jù)庫外文翻譯---關(guān)系數(shù)據(jù)庫的結(jié)構(gòu)-數(shù)據(jù)庫(編輯修改稿)
2025-02-24 11:55 本頁面
【文章內(nèi)容簡(jiǎn)介】 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 datamodeling 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 plete, it is repeated as many times as necessary to ensure that the designer is capturing what is important enough to go into the database. Let39。s take a closer look at each step in the datamodeling process. Defining the Entities First, 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 anization 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。s widget pany database, he identifies four entities: employees, departments, salary grades, and bonuses. These will bee 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 pany 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 binations of salary grades and departments. Associative Table:A database table that stores the valid binations of rows from two other tables and usually enforces a business rule. An associative table resolves a manytomany relationship. In general, there are three types of relationships in a relational database: Onetomany The most mon type of relationship is onetomany. 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 onetoma
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