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SELECT NAME, CITY, REGION FROM REPDATA WHERE SALES > QUOTA; NAME -----------Mary Jones Sam Clark Dan Roberts Paul Cruz Bill Adams Sue Smith Larry Fitch CITY -----------New York New York Chicago Chicago Atlanta Los Angeles Los Angeles REGION -------Eastern Eastern Eastern Eastern Eastern Western Western
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The name of the view, REPDATA, appears in the FROM clause just like a table name, and the columns of the view are referenced in the SELECT statement just like the columns of a real table. For some views, you can also use the INSERT, DELETE, and UPDATE statements to modify the data visible through the view, as if it were a real table. Thus, for all practical purposes, the view can be used in SQL statements as if it were a real table. However, it is essential to understand the implications to the underlying tables before updating views.
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When the DBMS encounters a reference to a view in a SQL statement, it finds the definition of the view stored in the database. Then the DBMS translates the request that references the view into an equivalent request against the source tables of the view and carries out the equivalent request. In this way, the DBMS maintains the illusion of the view while maintaining the integrity of the source tables. For simple views, the DBMS may construct each row of the view on the fly, drawing the data for the row from the source table(s). For more complex views, the DBMS must actually materialize the view; that is, the DBMS must actually carry out the query that defines the view and store its results in a temporary table. The DBMS fills your requests for view access from this temporary table and discards the table when it is no longer needed. Regardless of how the DBMS actually handles a particular view, the result is the same for the user the view can be referenced in SQL statements exactly as if it were a real table in the database.
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Views provide a variety of benefits and can be useful in many different types of databases. In a personal computer database, views are usually a convenience, defined to simplify database requests. In a commercial database installation, views play a central role in defining the structure of the database for its users and enforcing its security. Views provide these major benefits: Security Each user can be given permission to access the database only through a small set of views that contain the specific data the user is authorized to see, thus restricting the user s access to stored data. Query simplicity A view can draw data from several different tables and present it as a single table, turning multitable queries into single-table queries against the view.
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Database Structure
Structural simplicity Views can give a user a personalized view of the database structure, presenting the database as a set of virtual tables that make sense for that user. Insulation from change A view can present a consistent, unchanged image of the structure of the database, even if the underlying source tables are split, restructured, or renamed. Note, however, that the view definition must be updated whenever underlying tables or columns referenced by the view are renamed. Data integrity If data is accessed and entered through a view, the DBMS can automatically check the data to ensure that it meets specified integrity constraints.
Disadvantages of Views
While views provide substantial advantages, there are also three major disadvantages to using a view instead of a real table: Performance Views create the appearance of a table, but the DBMS must still translate queries against the view into queries against the underlying source tables. If the view is defined by a complex multitable query, then even a simple query against the view becomes a complicated join, and it may take a long time to complete. However, the issue isn t because the query is in a view any poorly constructed query can present performance problems the hazard is that the complexity is hidden in the view, and thus users are not aware of how much work the query is performing. Manageability Like all database objects, views must be managed. If developers and database users are allowed to freely create views without controls or standards, the DBA s job becomes that much more difficult. This is especially true when views are created that reference other views, which in turn reference even more views. The more layers between the base tables and the views, the more difficult it is to resolve problems attributed to the views. Update restrictions When a user tries to update rows of a view, the DBMS must translate the request into an update on rows of the underlying source tables. This is possible for simple views, but more complex views cannot be updated; they are read-only. These disadvantages mean that you cannot indiscriminately define views and use them instead of the source tables. Instead, you must in each case consider the advantages provided by using a view and weigh them against the disadvantages.
Creating a View (CREATE VIEW)
The CREATE VIEW statement, shown in Figure 14-2, is used to create a view. The statement assigns a name to the view and specifies the query that defines the view. To create the view successfully, you must have permission to access all of the tables referenced in the query. In some DBMSs (notably Oracle), you must also have permission to create views. The CREATE VIEW statement can optionally assign a name to each column in the newly created view. If a list of column names is specified, it must have the same number of items as the number of columns produced by the query. Note that only the column names are
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