vb.net 2d barcode dll Implementing Data Security in Java

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Implementing Data Security
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Last, but not least, views are a powerful means to implement data security. Views allow you to hide certain data from database users and applications. The view query precisely determines which rows and columns are exposed via the view. By using the GRANT and REVOKE commands on your views, you specify in detail which actions against the view data are allowed. In this approach, you don t grant any privileges at all on the underlying base tables, since you obviously don t want database users or applications to bypass the views and access the base tables directly.
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10.4 Data Manipulation via Views
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As you ve learned in this chapter, views are virtual tables, and they are supposed to behave like tables as much as possible. For retrieval, that s no problem. However, data manipulation via views is not always possible. A view is theoretically updatable if the DML command against the view can be unambiguously decomposed into corresponding DML commands against rows and columns of the underlying base tables. Let s consider the three views created in Listings 10-10 and 10-11. Listing 10-10. CRS_OFFERINGS View, Based on a Join SQL> create or replace view crs_offerings as 2 select o.course as course_code, c.description, o.begindate 3 from offerings o 4 join 5 courses c 6 on (o.course = c.code); View created. SQL> Listing 10-11. Simple EMP View and Aggregate AVG_EVALUATIONS View SQL> create or replace view emp as 2 select empno, ename, init 3 from employees; View created. SQL> create or replace view avg_evaluations as 2 select course 3 , avg(evaluation) as avg_eval
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View created. SQL> First, let s look at the most simple view: the EMP view. The Oracle DBMS should be able to delete rows from the EMPLOYEES table via this view, or to change any of the three column values exposed by the view. However, inserting new rows via this view is impossible, because the EMPLOYEES table has NOT NULL columns without a default value (such as the date of birth) outside the scope of the EMP view. See Listing 10-12 for some DML experiments against the EMP view. Listing 10-12. Testing DML Commands Against the EMP View SQL> delete from emp 2 where empno = 7654; 1 row deleted. SQL> update emp 2 set ename = 'BLACK' 3 where empno = 7698; 1 row updated. SQL> insert into emp 2 values (7999,'NEWGUY','NN'); insert into e * ERROR at line 1: ORA-01400: cannot insert NULL into ("BOOK"."EMPLOYEES"."BDATE") SQL> rollback; Rollback complete. SQL> Note that the ORA-01400 error message in Listing 10-12 actually reveals several facts about the underlying (and supposedly hidden) table: The schema name (BOOK) The table name (EMPLOYEES) The presence of a mandatory BDATE column
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Before you think you ve discovered a security breach in the Oracle DBMS, I should explain that you get this informative error message only because you are testing the EMP view while connected as BOOK. If you are connected as a different database user with INSERT privilege against the EMP view only, the error message becomes as follows: ORA-01400: cannot insert NULL into ( )
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Updatable Join Views
The CRS_OFFERINGS view (see Listing 10-10) is based on a join of two tables: OFFERINGS and COURSES. Nevertheless, you are able to perform some data manipulation via this view, as long as the data manipulation can be translated into corresponding actions against the two underlying base tables. CRS_OFFERINGS is an example of an updatable join view. The Oracle DBMS is getting closer and closer to the full implementation of Ted Codd s rule 6 (see 1). Listing 10-13 demonstrates testing some DML commands against this view. Listing 10-13. Testing DML Commands Against the CRS_OFFERINGS View SQL> delete from crs_offerings where course_code = 'ERM'; 1 row deleted. SQL> insert into crs_offerings (course_code, begindate) 2 values ('OAU' , trunc(sysdate)); 1 row created. SQL> rollback; Rollback complete. SQL> There are some rules and restrictions that apply to updatable join views. Also, the concept of keypreserved tables plays an important role in this area. As the name indicates, a key-preserved table is an underlying base table with a one-to-one row relationship with the rows in the view, via the primary key or a unique key. These are some examples of updatable join view restrictions: You are allowed to issue DML commands against updatable join views only if you change a single underlying base table. For INSERT statements, all columns into which values are inserted must belong to a key-preserved table. For UPDATE statements, all columns updated must belong to a key-preserved table. For DELETE statements, if the join results in more than one key-preserved table, the Oracle DBMS deletes from the first table named in the FROM clause. If you created the view using WITH CHECK OPTION, some additional DML restrictions apply, as explained a little later in this section.
As you can see in Listing 10-13, the DELETE and INSERT statements against the CRS_OFFERINGS updatable join view succeed. Feel free to experiment with other data manipulation commands. The Oracle error messages are self-explanatory if you hit one of the restrictions: ORA-01732: data manipulation operation not legal on this view ORA-01752: cannot delete from view without exactly one key-preserved table ORA-01779: cannot modify a column which maps to a non key-preserved table
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