Đang chuẩn bị liên kết để tải về tài liệu:
Database Design Using Entity-Relationship Diagrams phần 6

Thay thế bằng 1: M mối quan hệ Bây giờ chúng ta có một thực thể HỌC SINH HỌC + (một thực thể giao nhau), mô tả ngữ pháp của chúng ta về thực thể giao nhau này sẽ là: Không có các phím ứng cử viên (giao nhau thực thể): HỌC SINH + HỌC thực thể không có một phím ứng cử viên của riêng mình, nhưng đúng hơn | Replaced with Two 1 M Relationships Now that we have a STUDENT COURSE entity an intersecting entity our grammatical description of this intersecting entity would be The Entity This database records data about STUDENT-COURSE combinations STUDENT COURSE. For each STUDENT COURSE in the database we record a grade. The Attributes For each STUDENT COURSE there always will be one and only one grade for each STUDENT COURSE combination. The value for grade will not be subdivided. The Keys d No candidate keys intersecting entity The STUDENT COURSE entity does not have a candidate key of its own but rather each STUDENT COURSE entity will be identified by keys belonging to the STUDENT and COURSE entities. The latter statement is very close and for a user hopefully indistinguishable from the key statements found in the attribute on a relationship grammar above For the relationship between STUDENT and COURSE we will record a grade. The grade depends on both entities STUDENT and COURSE for identification. More Entities and Relationships In the handling of a database we have to model the information presented. We will likely have situations that call for more than two entities and more than one binary relationship. Again a binary relationship is a relationship between two entities. Chapter 7 looks at ternary and higher relationship combinations. This section deals with situations where the information about the database indicates that we have to expand our diagrams with more entities but all the connections will be binary. More than Two Entities Let us again reconsider the STUDENT-COURSE ER diagram Figure 6.1. If this database were oriented toward a college the courses would have instructors and the instructors would be related to the courses. We would consider adding INSTRUCTOR to our database per our methodology steps 4 and 5 which say Step 4 If another entity is appropriate draw the second entity with its attributes. Repeat step 2 to see if this entity should be further split .