tailieunhanh - Database Design Using Entity-Relationship Diagrams phần 2

Thời gian này, tuy nhiên, chúng tôi sẽ giữ cho SqlDataAdapter xung quanh và giữ cho nó ràng buộc với DataSet. DataSet mới này sẽ được gọi là "Thạc sĩ DataSet". Tất cả các cột trong hàng áp dụng trong tập hợp dữ liệu thay đổi sẽ được sao chép vào cột phù hợp trong tập hợp dữ liệu tổng thể. | Chapter 2 The Basic ER Diagram A Data Modeling Schema This chapter begins by describing a data modeling approach and then introduces entity relationship ER diagrams. The concept of entities attributes relationships and keys are introduced. The first three steps in an ER design methodology are developed. Step 1 begins by building a one-entity diagram. Step 2 concentrates on using structured English to describe a database. Step 3 the last section in this chapter discusses mapping the ER diagram to a relational database. These concepts the diagram structured English and mapping will evolve together as the book progresses. At the end of the chapter we also begin a running case study which will be continued at the ends of the subsequent chapters. What Is a Data Modeling Schema A data modeling schema is a method that allows us to model or illustrate a database. This device is often in the form of a graphic diagram but other means of communication are also desirable non computer-people may or may not understand diagrams and graphics. The ER diagram ERD is a graphic tool that facilitates data modeling. The ERD is a subset of semantic models in a database. Semantic models refer to models that intend to elicit meaning from data. ERDs are not the only semantic modeling tools but they are common and popular. When we begin to discuss the contents of a database the data model helps to decide which piece of data goes with which other piece of data on a conceptual level. An early concept in databases is to recognize that there are levels of abstraction that we can use in discussing databases. For example if we were to discuss the filing of names we could discuss this Abstractly that is we will file names of people we know. or Concretely that is we will file first middle and last names 20 characters each of people we know so that we can retrieve the names in alphabetical order on last name and we will put this data in a spreadsheet format on package x. If a person is designing a .