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14 rows selected. SQL> Listing 10-3 shows how you can transform this query into a view definition, by inserting one additional line at the beginning of the command. Listing 10-3. Creating a View from the Query in Listing 10-2 SQL> 2 3 4 5 6 7 8 9 10 11 create select , , , , , from view empdept_v as -- This line is added e.empno e.ENAME e.init d.dname d.location m.ENAME as MANAGER employees e join departments d using (deptno) join
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m on (m.empno = d.mgr);
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View created. SQL> This view is now a permanent part of your collection of database objects. However, note that if we had not used an alias for m.ENAME, Listing 10-3 would give the following Oracle error message: ORA-00957: duplicate column name
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Getting Information About Views from the Data Dictionary
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Listing 10-4 queries the USER_OBJECTS data dictionary view. As you can see, you now have two views in your schema: DEPT20_V and EMPDEPT_V. Listing 10-4. Querying the Data Dictionary to See Your Views SQL> 2 3 4 select from where order object_name, object_type user_objects object_type in ('TABLE','VIEW') by object_type, object_name; OBJECT_TYPE ----------TABLE TABLE TABLE TABLE TABLE TABLE TABLE TABLE TABLE VIEW VIEW
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OBJECT_NAME -----------------------------COURSES DEPARTMENTS DEPT20_T E EMPLOYEES HISTORY OFFERINGS REGISTRATIONS SALGRADES DEPT20_V EMPDEPT_V 11 rows selected. SQL>
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Listing 10-5 shows that you can use the SQL*Plus DESCRIBE command on a view, just as you can on regular tables, and it also shows an example of a query against a view.
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Listing 10-5. Using DESCRIBE and Writing Queries Against Views SQL> describe empdept_v Name ----------------------------EMPNO ENAME INIT DNAME LOCATION MANAGER Null -------NOT NULL NOT NULL NOT NULL NOT NULL NOT NULL NOT NULL Type ------------NUMBER(4) VARCHAR2(8) VARCHAR2(5) VARCHAR2(10) VARCHAR2(8) VARCHAR2(8)
SQL> select * from empdept_v where manager = 'CLARK'; EMPNO -------7934 7839 7782 SQL> You can query the USER_VIEWS data dictionary view to retrieve your view definitions, as shown in Listing 10-6. ENAME -------MILLER KING CLARK INIT ----TJA CC AB DNAME ---------ACCOUNTING ACCOUNTING ACCOUNTING LOCATION -------NEW YORK NEW YORK NEW YORK MANAGER -------CLARK CLARK CLARK
Note The two leading SQL*Plus commands in Listing 10-6 are used only to make the results more readable. 11 discusses these (and many other) SQL*Plus commands in more detail.
Listing 10-6. Retrieving View Definitions from the Data Dictionary SQL> set long 999 SQL> column text format a42 word wrapped SQL> select view_name, text 2 from user_views; VIEW_NAME TEXT ------------------------------ -----------------------------------------DEPT20_V select "EMPNO","ENAME","INIT","JOB", "MGR","BDATE","MSAL","COMM","DEPTNO" from employees where deptno=20 EMPDEPT_V select , , , e.empno e.ENAME e.init d.dname
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SQL> Apparently, if you define a view with a query starting with SELECT * FROM ..., the asterisk (*) gets expanded (and stored) as a comma-separated list of column names. Compare the query in Listing 10-1, where you created the DEPT20_V view, with the TEXT column contents in Listing 10-6.
Replacing and Dropping Views
You cannot change the definition of an existing view. Oracle SQL offers an ALTER VIEW command, but you can use that command only to recompile views that became invalid. You can drop a view definition only, with the DROP VIEW command. The DROP VIEW command is very straightforward, and doesn t need additional explanation: SQL> drop view <view_name>; Alternatively, you can replace the definition of an existing view with the CREATE OR REPLACE VIEW command, as described earlier in this section.
10.3 What Can You Do with Views
You can use views for many different purposes. This section lists and explains the most important ones: to simplify database retrieval, to maintain logical data independence, and to implement data security.
Simplifying Data Retrieval
Views can simplify database retrieval significantly. You can build up (and test) complex queries step by step, for more control over the correctness of your queries. In other words, you will be more confident that your queries return the right results. You can also store (hide) frequently recurring standard queries in a view definition, thus reducing the number of unnecessary mistakes. For example, you might define views based on frequently joined tables, UNION constructs, or complex GROUP BY statements. Suppose we are interested in an overview showing all employees who have attended more course days than the average employee. This is not a trivial query, so let s tackle it in multiple phases. As a first step toward the final solution, we ask the question, How many course days did everyone attend The query in Listing 10-7 provides the answer.
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