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SQL: A Beginner s Guide
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You discuss creating views and dropping views, but you do not mention altering views. Does SQL support any sort of ALTER VIEW statement No, the SQL:2006 standard does not support altering views. However, some RDBMSs support an ALTER VIEW statement. Be aware, though, that the functionality supported by these statements can vary from product to product. For example, SQL Server and MySQL have ALTER VIEW statements that are fairly robust and allow you to change many aspects of the view definition, including the SELECT statement. On the other hand, the ALTER VIEW statement in Oracle is used to manually recompile a view to avoid runtime overhead or to modify certain constraints that Oracle supports on views. To actually alter an Oracle view, you must first drop it and then recreate it, as is the case with the SQL standard. However, Oracle has a CREATE OR REPLACE VIEW statement that essentially allows you to drop and recreate a view in a single step. In the examples that you use to show how views are created, you use one or two tables for your source data. Can views be based on more than two tables Yes, a view can be based on as many tables as can be logically queried in the SELECT statement. For example, suppose you want to create a view in the INVENTORY database. (The INVENTORY database is the one you ve been working with for the Try This exercises in this book.) The view might display artists names alongside CD titles. To do that, however, your SELECT statement would have to join together three tables. You would have to match the ARTIST_ID values in the ARTISTS table and the ARTIST_CDS table, and you would have to match the COMPACT_DISC_ID values in the COMPACT_ DISCS table and the ARTIST_CDS table. The result would be a view that displays a list of artists and their CDs. (In 11, I will discuss how you can join these tables together in your SELECT statement.) In the examples that you use to show how views are created, all the views reference base tables. Are all views created only on base tables No, views can be created using query expressions that pull data from other views. Also, it is possible to create a view that contains only calculated data and thus has no data that maps back to a base table.
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In this Try This exercise, you will create two views in the INVENTORY database. The views will be based on tables you created in previous Try This exercises. The first view will be based on a single table, and the second view will be based on two tables. You ll create the second view two different times. You ll create it once, then drop the view definition from the database, and then recreate a modified version of the view. You can download the Try_This_05.txt file, which contains the SQL statements used in this exercise.
Step by Step
1. Open the client application for your RDBMS and connect to the INVENTORY database. 2. The first view that you ll create is named CDS_IN_STOCK. The view is based on the
CD_TITLE and IN_STOCK columns in the COMPACT_DISCS table. You want the view to include only those rows whose values in the IN_STOCK column are greater than 10. The view will use the same column names as the table and will include the WITH CHECK OPTION to prevent values less than or equal to 10 from being added to the IN_STOCK column. Enter and execute the following SQL statement:
CREATE VIEW CDS_IN_STOCK AS SELECT CD_TITLE, IN_STOCK FROM COMPACT_DISCS WHERE IN_STOCK > 10 WITH CHECK OPTION; 3. Next, you will create a view named CD_PUBLISHERS that will contain the CD_TITLE
column and the PUBLISHER column. The view will be based on the CD_TITLE column in the COMPACT_DISCS table and the COMPANY_NAME column of the CD_LABELS table. You will need to use a WHERE clause to match rows in the two tables. The WHERE clause will also limit the rows included in the view to those whose LABEL_ID value in the CD_LABELS table is either 5403 or 5402. Enter and execute the following SQL statement:
CREATE VIEW CD_PUBLISHERS ( CD_TITLE, PUBLISHER ) AS SELECT COMPACT_DISCS.CD_TITLE, CD_LABELS.COMPANY_NAME FROM COMPACT_DISCS, CD_LABELS WHERE COMPACT_DISCS.LABEL_ID = CD_LABELS.LABEL_ID AND CD_LABELS.LABEL_ID = 5403 OR CD_LABELS.LABEL_ID = 5402; 4. You decide that you do not want to limit the rows to specific values in the LABEL_ID
column, so you must drop the view definition from the database and recreate the view without the value restrictions. Enter and execute the following SQL statement:
DROP VIEW CD_PUBLISHERS;
(continued)
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