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Mastering Microsoft Visual Basic 2008 phần 8

Gần đây tôi đã chạy vào một số nhiệm vụ liên quan đến trang web có giá trị đề cập đến ở đây. Một chương trình cho việc đặt hình chìm trên hình ảnh là người đầu tiên. Watermark là một hình ảnh được đặt trên một hình ảnh để chỉ ra nguồn gốc của nó. Watermark là minh bạch, vì vậy nó không làm mờ hình ảnh, | 770 chapter 21 basic concepts of relational databases tables and set up relations between them. And only then can we code against the database. It s not uncommon to add a table to an existing database at a later stage but this reveals some flaw in the initial database design. Working with Relationships Indices and Constraints To manipulate relationships indices and constraints open one of the tables in design mode. The Table Designer menu contains a list of tasks you can perform with the table including changing the relationships indexes and constraints. RELATIONSHIPS Relationships are the core of a relational database because they relate tables to one another. To create a relationship double-click a table s name in Server Explorer and then choose Table Designer Relationships which displays the Foreign Key Relationships dialog box shown in Figure 21.9. This figure shows that there is already a relationship between the Categories table and the Products table. The relationship is called FK_Products_Categories and it relates the primary and foreign keys of the two tables field CategorylD . The names of the two related tables appear in two read-only boxes. When you create a new relationship you can select a table from a drop-down list. Under each table s name you see a list of fields. Here you select the matching fields in the two tables. Most relationships are based on a single field which is common to both tables. However you can relate two tables based on multiple fields in which case all pairs must match in a relationship . The check boxes at the bottom of the page specify how the DBMS will handle the relationship they are discussed shortly . FIGURE 21.9 The Foreign Key Relationships dialog box for the Categories table To create a new relationship with another table click the Add button. A new relationship will be added with a default name which you can change. Like all other objects relationships have unique names too. Expand the Tables And Columns Specification .