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Views and SQL Security
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In addition to the restrictions on table access provided by the SQL privileges, views also play a key role in SQL security. By carefully defining a view and giving a user permission to access the view but not its source tables, you can effectively restrict the user s access to only selected columns and rows. Views thus offer a way to exercise very precise control over what data is made visible to which users. For example, suppose you wanted to enforce this security rule in the sample database: Accounts receivable personnel should be able to retrieve employee numbers, names, and office numbers from the SALESREPS table, but data about sales and quotas should not be available to them. You can implement this security rule by defining a view as follows:
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CREATE VIEW REPINFO AS SELECT EMPL_NUM, NAME, REP_OFFICE FROM SALESREPS DATABASE STRUCTURE
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and giving the SELECT privilege for the view to the ARUSER user-id, as shown in Figure 15-3. This example uses a vertical view to restrict access to specific columns.
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SQL: The Complete Reference
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Figure 15-3.
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Using a view to restrict column access
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Horizontal views are also effective for enforcing security rules such as this one: The sales managers in each region should have full access to SALESREPS data for the salespeople assigned to that region. As shown in Figure 15-4, you can define two views, EASTVIEWS and WESTVIEWS, containing SALESREPS data for each of the two regions, and then grant each office manager access to the appropriate view.
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Figure 15-4.
Using views to restrict row access
Of course, views can be much more complex than the simple row and column subsets of a single table shown in these examples. By defining a view with a grouped query, you can give a user access to summary data but not to the detailed rows in the underlying table. A view can also combine data from two or more tables, providing precisely the data needed by a particular user and denying access to all other data. The usefulness of views for implementing SQL security is limited by the two fundamental restrictions described earlier in 14: I Update restrictions. The SELECT privilege can be used with read-only views to limit data retrieval, but the INSERT, DELETE, and UPDATE privileges are meaningless for these views. If a user must update the data visible in a read-only view, the user must be given permission to update the underlying tables and must use INSERT, DELETE, and UPDATE statements that reference those tables. I Performance. Because the DBMS translates every access to a view into a corresponding access to its source tables, views can add significant overhead to database operations. Views cannot be used indiscriminately to restrict database access without causing overall database performance to suffer.
DATABASE STRUCTURE
SQL: The Complete Reference
Granting Privileges (GRANT)
The basic GRANT statement, shown in Figure 15-5, is used to grant security privileges on database objects to specific users. Normally, the GRANT statement is used by the owner of a table or view to give other users access to the data. As shown in the figure, the GRANT statement includes a specific list of the privileges to be granted, the name of the table to which the privileges apply, and the user-id to which the privileges are granted. The GRANT statement shown in the syntax diagram conforms to the ANSI/ISO SQL standard. Many DBMS brands follow the DB2 GRANT statement syntax, which is more flexible. The DB2 syntax allows you to specify a list of user-ids and a list of tables, making it simpler to grant many privileges at once. Here are some examples of simple GRANT statements for the sample database: Give order-processing users full access to the ORDERS table.
GRANT SELECT, INSERT, DELETE, UPDATE ON ORDERS TO OPUSER
Figure 15-5.
The GRANT statement syntax diagram
15:
SQL Security
Let accounts receivable users retrieve customer data and add new customers to the CUSTOMERS table, but give order-processing users read-only access.
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