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First of all, if you create a view with the WITH READ ONLY option (see Figure 10-1), data manipulation via that view is impossible by definition, regardless of how you defined the view. The AVG_EVALUATIONS view definition (see Listing 10-11) contains a GROUP BY clause. This implies that there is no longer a one-to-one relationship between the rows of the view and the rows of the underlying base table. Therefore, data manipulation via the AVG_EVALUATIONS view is impossible. If you use SELECT DISTINCT in your view definition, this has the same effect: it makes your view nonupdatable. You should try to avoid using SELECT DISTINCT in view definitions, because it has additional disadvantages; for example, each view access will force a sort to take place, whether or not you need it. The set operators UNION, MINUS, and INTERSECT also result in nonupdatable views. For example, imagine that you are trying to insert a row via a view based on a UNION in which underlying base table should the DBMS insert that row The Oracle documentation provides all of the details and rules with regard to view updatability. Most rules and exceptions are rather straightforward, and as noted earlier, most Oracle error messages clearly indicate the reason why certain data manipulation commands are forbidden. The data dictionary offers a helpful view to find out which of your view columns are updatable: the USER_UPDATABLE_COLUMNS view. For example, Listing 10-14 shows that you cannot do much with the DESCRIPTION column of the CRS_OFFERINGS view. This is because it is based on a column from the COURSES table, which is a not a key-preserved table in this view. Listing 10-14. View Column Updatability Information from the Data Dictionary SQL> 2 3 4 select , from where column_name updatable, insertable, deletable user_updatable_columns table_name = 'CRS_OFFERINGS'; UPD --YES NO YES INS --YES NO YES DEL --YES NO YES
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COLUMN_NAME -------------------COURSE_CODE DESCRIPTION BEGINDATE SQL>
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In a chapter about views, it s worth mentioning that PL/SQL (the standard procedural programming language for Oracle databases) provides a way to make any view updatable. With PL/SQL, you can define instead-of triggers on your views. These triggers take over control as soon as any data manipulation commands are executed against the view. This means that you can make any view updatable, if you choose, by writing some procedural PL/SQL code. Obviously, it is your sole responsibility to make sure that those instead-of triggers do the right things to your database to maintain data consistency and integrity. Instead-of triggers should not be your first thought to solve data manipulation issues with views. However, they may solve your problems in some special cases, or they may allow you to implement a very specific application behavior.
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If data manipulation is allowed via a certain view, there are two rather curious situations that deserve attention: You change rows with an UPDATE command against the view, and then the rows don t show up in the view anymore. You add rows with an INSERT command against the view; however, the rows don t show up when you query the view.
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Do you still have the DEPT20_V view, created in Listing 10-1 Check out what happens in Listing 10-15: by updating four rows, they disappear from the view. Listing 10-15. UPDATE Makes Rows Disappear SQL> select * from dept20_v; EMPNO ----7369 7566 7788 7876 7902 ENAME -------SMITH JONES SCOTT ADAMS FORD INIT ----N JM SCJ AA MG JOB MGR BDATE MSAL COMM DEPTNO -------- ----- ----------- ----- ----- -----TRAINER 7902 17-DEC-1965 800 20 MANAGER 7839 02-APR-1967 2975 20 TRAINER 7566 26-NOV-1959 3000 20 TRAINER 7788 30-DEC-1966 1100 20 TRAINER 7566 13-FEB-1959 3000 20
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