Database Reference
In-Depth Information
Figure 12.20. View Specifications sheet for the VENDOR PRODUCT COUNT view
You then repeat this process for every view you've identified for Mike's database.
Summary
We began this chapter with a definition of a view, and you learned that it is a virtual table
that does not contain or store data. Views are useful for several reasons—they provide a
means for you to work with data from multiple tables, they help enforce data integrity, and
they help keep data secure or confidential.
We then discussed the three types of views:
data, aggregate,
and
validation.
You learned
that each type of view could be based on one or more tables, other views, or a combination
of both. Your RDBMS will rebuild and repopulate a view every time you access it, using
the most current data from the view's base tables. As you now know, there must be rela-
tionships between tables in a multitable view (thus making the view's information valid
and meaningful), and the characteristics of those relationships are carried forth through the
view. Additionally, you can modify most views, and all the modifications you make to the
data are passed through the view to the base tables. You also learned that
validation views
work in the same manner as validation tables and that they have distinct advantages over
validation tables. For instance, validation views can incorporate data from multiple tables.
The chapter then continued with a discussion of determining and defining views for the
database. Here you learned several specific points to keep in mind while you work with
users and management to identify the organization's view requirements. Next, we dis-
cussed how to define a view, and you learned how to create a
view diagram
to document
the view. Now you know how to select fields from the base tables and assign them to the
view.
We then discussed how to use calculated fields in a view. You learned that you could use
them to help provide pertinent information and to enhance how the view displays its data.
You also learned that calculated fields are especially crucial in aggregate views and that
each calculated field uses an expression to derive the value it displays. Next, you learned
how to apply a filter to a view so that it will retrieve and display a specific set of records.
The view will display a given record only if it meets the criteria you've imposed against
one or more fields in the view. You frame each criterion as an expression and use it to test
the value of a particular field.
The chapter closed with a discussion of the View Specifications sheet. Here you learned
how to document the characteristics of the view, such as its name and type. You also
learned about the items that compose the View Specifications sheet and how you use them
to record the view's characteristics.
