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Creating a View from a Query
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Listing 10-2 shows an example of a regular query with its result. The query is a join over three tables, providing information about all employees and their departments. Note that we use an alias in the SELECT clause (see line 6) to make sure that all columns in the query result have different names. See line 2, where you select the ENAME column, too. Listing 10-2. A Regular Query, Joining Three Tables SQL> 2 3 4 5 6 7 8 9 10 11 select , , , , , from e.empno e.ENAME e.init d.dname d.location m.ENAME as MANAGER employees e join departments d using (deptno) join employees m on (e.empno = d.mgr); INIT ----N JAM TF JM P R AB SCJ CC JJ AA R MG TJA DNAME ---------TRAINING SALES SALES TRAINING SALES SALES ACCOUNTING TRAINING ACCOUNTING SALES TRAINING SALES TRAINING ACCOUNTING LOCATION -------DALLAS CHICAGO CHICAGO DALLAS CHICAGO CHICAGO NEW YORK DALLAS NEW YORK CHICAGO DALLAS CHICAGO DALLAS NEW YORK MANAGER ------JONES BLAKE BLAKE JONES BLAKE BLAKE CLARK JONES CLARK BLAKE JONES BLAKE JONES CLARK
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EMPNO -------7369 7499 7521 7566 7654 7698 7782 7788 7839 7844 7876 7900 7902 7934
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ENAME -------SMITH ALLEN WARD JONES MARTIN BLAKE CLARK SCOTT KING TURNER ADAMS JONES FORD MILLER
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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.
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Listing 10-3. Creating a View from the Query in Listing 10-2 SQL> 2 3 4 5 6 7 8 9 10 11 12 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 employees 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 OBJ 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 obj object_type in ('TABLE','VIEW') by object_type, object_name; OBJECT_TYPE ----------TABLE TABLE TABLE TABLE TABLE TABLE TABLE TABLE TABLE
OBJECT_NAME -----------------------------COURSES DEPARTMENTS DEPT20_T E EMPLOYEES HISTORY OFFERINGS REGISTRATIONS SALGRADES
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DEPT20_V EMPDEPT_V 11 rows selected. SQL>
VIEW VIEW
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. 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;
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VIEW_NAME TEXT ------------------------------ -----------------------------------------DEPT20_V select "EMPNO","ENAME","INIT","JOB", "MGR","BDATE","MSAL","COMM","DEPTNO" from employees where deptno=20 EMPDEPT_V select , , , , , from e.empno e.ENAME e.init d.dname d.location m.ENAME as MANAGER employees e join departments d using (deptno) join employees m on (m.empno = d.mgr)
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.
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