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Implementing a security scheme and enforcing security restrictions are the responsibility of the DBMS software. The SQL 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: 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. 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. 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: Let Sam Clark retrieve and insert data in the OFFICES table.
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GRANT SELECT, INSERT ON OFFICES TO SAM;
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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 Once granted, the privileges can be rescinded later with this REVOKE statement: Take away the privileges granted earlier to Sam Clark.
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REVOKE SELECT, INSERT ON OFFICES FROM SAM;
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The GRANT and REVOKE statements are described in detail later in this chapter, in the sections Granting Privileges (GRANT) and Revoking Privileges (REVOKE).
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User-Ids
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Each user of a SQL-based database is typically assigned a user-id, a short name that identifies the user to the DBMS software. The user-id is at the heart of SQL security. Every SQL statement executed by the DBMS is carried out on behalf of a specific user-id. The user-id determines
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SQL Security
Order processing dept.
Accounts receivable dept.
Full access ORDERS Table
SELECT, UPDATE INSERT, SELECT some some SELECT columns columns CUSTOMERS Table
Larry Fitch, Los Angeles office manager
SALESREPS Table SELECT some rows
OFFICES Table Available to all users Full access to all data Full access to all data
SELECT some rows Bob Smith, Chicago office manager
Sam Clark V.P. Sales
George Watkins V.P. Marketing
PART IV
FIGURE 15-1
A security scheme for the sample database
whether the statement will be permitted or prohibited by the DBMS. In a commercial database, user-ids either are assigned by the database administrator, or, for products that can use external identifiers from operating system accounts, by a security administrator. A personal computer database may have only a single user-id, identifying the user who created and who owns the database. Special-purpose databases (for example, those designed to be embedded within an application or in a special-purpose system), may not need the additional overhead associated with SQL security. These databases typically operate as if there were a single user-id. In practice, the restrictions on the names that can be chosen as user-ids vary from implementation to implementation. The SQL1 standard permitted user-ids of up to 18 characters and required them to be valid SQL names, which was increased to up to 128 characters in later versions. In some mainframe DBMS systems, user-ids may have no more than eight characters. In Sybase and SQL Server, user-ids may have up to 30 characters. If portability is a concern, it s best to limit user-ids to eight or fewer characters. Figure 15-2 shows various users who need access to the sample database and the typical user-ids assigned to them. Note that all of the users in the order-processing department can be assigned the same user-id because they are to have identical privileges in the database. However, most security experts recommend against such a practice because of
Part IV:
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