c# barcode image generation library C H A P T E R 10 in Java

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C H A P T E R 10
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This chapter covers views, a very important component of the relational model (see Ted Codd s rule 6, in 1). The first section explains the concept of views. The second section discusses how to use the CREATE VIEW command to create views. In the next section, you ll learn about the various ways you can use views in SQL, in the areas of retrieval, logical data independency, and security. Then we explore the (im)possibilities of data manipulation via views. How does it work, which are the constraints, and what should we consider You ll learn about updatable views, nonupdatable views, and the WITH CHECK OPTION clause of the CREATE VIEW command. Section 10.5 discusses data manipulation via inline views. This name is slightly confusing, because inline views are not real views. Rather, they are subqueries in the FROM clause, as discussed in the previous chapter. Data manipulation via inline views allows you to perform various complicated and creative data manipulation operations, which would otherwise be very complex (or impossible) via the underlying base tables. Section 10.6 covers views and performance. Following that is a section about materialized views. Materialized views are very popular in data warehousing environments, which have relatively high data volumes with mainly read-only access. Materialized views allow you to improve query response times with some form of controlled redundancy.
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10.1 What Are Views
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The result of a query is always a table, or more precisely, a derived table. Compared with real tables in the database, the result of a query is volatile, but nevertheless, the result is a table. The only thing that is missing for the query result is a name. Essentially, a view is nothing more than a query with a given name. A more precise definition is as follows:
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A view is a virtual table with the result of a stored query as its contents, which are derived each time you access the view.
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The first part of this definition states two things: A view is a virtual table: That is, you can treat a view (in almost all circumstances) as a table in your SQL statements. Every view has a name, and that s why views are also referred to as named queries. Views have columns, each with a name and a datatype, so you can execute queries against views, and you can manipulate the contents of views (with some restrictions) with INSERT, UPDATE, DELETE, and MERGE commands. A view is a virtual table: In reality, when you access a view, it only behaves like a table. Views don t have any rows; that s why the view definition says contents
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(within quotation marks). You define views as named queries, which are stored in the data dictionary; that s why another common term for views is stored queries. Each time you access the contents of a view, the Oracle DBMS retrieves the view query from the data dictionary and uses that query to produce the virtual table. Data manipulation on a view sounds counterintuitive; after all, views don t have any rows. Nevertheless, views are supposed to behave like tables as much as possible. If you issue data manipulation commands against a view, the DBMS is supposed to translate those commands into corresponding actions against the underlying base tables. Note that some views are not updatable; that s why Ted Codd s rule 6 (see 1) explicitly refers to views being theoretically updatable. We ll discuss data manipulation via views in Section 10.4 of this chapter. Views are not only dependent on changes in the contents of the underlying base tables, but also on certain changes in the structure of those tables. For example, a view doesn t work anymore if you drop or rename columns of the underlying tables that are referenced in the view definition.
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