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Views allow you to redefine the structure of a database, giving each user a personalized view of the database structure and contents: I A view is a virtual table defined by a query. The view appears to contain rows and columns of data, just like a real table, but the data visible through the view is, in fact, the results of the query. I A view can be a simple row/column subset of a single table, it can summarize a table (a grouped view), or it can draw its data from two or more tables (a joined view). I A view can be referenced like a real table in a SELECT, INSERT, DELETE, or UPDATE statement. However, more complex views cannot be updated; they are read-only views. I Views are commonly used to simplify the apparent structure of a database, to simplify queries, and to protect certain rows and/or columns from unauthorized access. I Materialized views can improve the efficiency of database processing in situations where there is a very high volume of query activity and relatively low update activity.
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hen you entrust your data to a database management system, the security of the stored data is a major concern. Security is especially important in an SQL-based DBMS because interactive SQL makes database access very easy. The security requirements of a typical production database are many and varied:
I The data in any given table should be accessible to some users, but access by other users should be prevented. I Some users should be allowed to update data in a particular table; others should be allowed only to retrieve data. I For some tables, access should be restricted on a column-by-column basis. I Some users should be denied interactive SQL access to a table but should be allowed to use application programs that update the table. The SQL security scheme described in this chapter provides these types of protection for data in a relational database.
SQL Security Concepts
Implementing a security scheme and enforcing security restrictions are the responsibility of the DBMS software. The SQL language defines an overall framework for database security, and SQL statements are used to specify security restrictions. The SQL security scheme is based on three central concepts: I Users. The actors in the database. Each time the DBMS retrieves, inserts, deletes, or updates data, it does so on behalf of some user. The DBMS permits or prohibits the action depending on which user is making the request. I Database objects. The items to which SQL security protection can be applied. Security is usually applied to tables and views, but other objects such as forms, application programs, and entire databases can also be protected. Most users will have permission to use certain database objects but will be prohibited from using others. I Privileges. The actions that a user is permitted to carry out for a given database object. A user may have permission to SELECT and INSERT rows in a certain table, for example, but may lack permission to DELETE or UPDATE rows of the table. A different user may have a different set of privileges. Figure 15-1 shows how these security concepts might be used in a security scheme for the sample database. To establish a security scheme for a database, you use the SQL GRANT statement to specify which users have which privileges on which database objects. For example, here is a GRANT statement that lets Sam Clark retrieve and insert data in the OFFICES table of the sample database:
15:
SQL Security
Figure 15-1.
A security scheme for the sample database
DATABASE STRUCTURE
Let Sam Clark retrieve and insert data in the OFFICES table.
GRANT SELECT, INSERT ON OFFICES TO SAM
The GRANT statement specifies a combination of a user-id (SAM), an object (the OFFICES table), and privileges (SELECT and INSERT). Once granted, the privileges can be rescinded later with this REVOKE statement: Take away the privileges granted earlier to Sam Clark.
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