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SQL: A Beginner s Guide
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use the WITH GRANT OPTION, EmmaW will not be able to grant the privilege to another user. Incidentally, most security experts recommend that you never use this option because you quickly lose control over who has which privileges. Now that we ve taken a look at the syntax, let s look at a few examples. In the first example, we ll look at a GRANT statement that grants the SELECT privilege to the PUBLIC authorization identifier. The privilege is granted on a view named AVAILABLE_CDS, which lists the CDs that you currently have in stock. To grant the privilege, use the following statement:
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GRANT SELECT ON TABLE AVAILABLE_CDS TO PUBLIC;
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The SELECT privilege allows all database users (PUBLIC) to view data in the AVAILABLE_CDS view. However, because PUBLIC has not been granted any other privileges, users can view the data, but not take any action. In addition, because the WITH GRANT OPTION clause is not included in the statement, users cannot assign the SELECT privilege to any other users (which is a moot point in this case because everyone can already access the AVAILABLE_CDS view). Now let s look at another example. This time, I m granting the SELECT, UPDATE, and INSERT privileges to the SALES role and the ACCOUNTING role so that they have access to the CD_INVENTORY table:
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GRANT SELECT, UPDATE, INSERT ON TABLE CD_INVENTORY TO SALES, ACCOUNTING WITH GRANT OPTION;
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Notice that the privileges are separated by commas, as are the roles. As a result of this statement, the users associated with the SALES role and the ACCOUNTING role can view, update, and insert information into the CD_INVENTORY table. In addition, these users can assign the SELECT, UPDATE, and INSERT privileges to other users who need to access the CD_INVENTORY table. The next example we will examine is a slight variation on this last one. Everything is the same, except that this time, I specify which column can be updated:
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GRANT SELECT, UPDATE(CD_TITLE), INSERT ON TABLE CD_INVENTORY TO SALES, ACCOUNTING WITH GRANT OPTION;
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Notice that you can add a column name after the specific privilege. You can add column names only to the SELECT, INSERT, UPDATE, and REFERENCES privileges. If you add more than one column name, you must separate them with commas. The GRANT statement in this example still allows the Sales and Accounting users to view and insert information into the CD_INVENTORY table, but they can only update values in the CD_TITLE column. They cannot update any other column values in the table. In addition, although they can still assign privileges to other users, they can assign the UPDATE privilege only on the CD_TITLE column.
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Let s take a look at one more example that grants SELECT privileges to the PUBLIC authorization identifier:
GRANT SELECT(CD_TITLE, IN_STOCK) ON CD_INVENTORY TO PUBLIC;
The PUBLIC authorization identifier allows all users to view data in the CD_TITLE and IN_ STOCK columns of the CD_INVENTORY table, but they cannot view any other information in that table and they cannot modify the data in any way. Notice in this statement that the keyword TABLE isn t included. As I said earlier, TABLE is not required. The GRANT statement, when used in conjunction with the available privileges and the authorization identifiers, provides a strong foundation for your database security. However, each SQL implementation is different with regard to how security is implemented and maintained. Therefore, when it comes to matters of security, it is important that you work closely with network and database administrators and carefully read the product documentation.
Revoking Privileges
Now that you know how to grant privileges to authorization identifiers, it s time to learn how to revoke those privileges. The statement that you use to revoke privileges is the REVOKE statement, as shown in the following syntax: REVOKE [ GRANT OPTION FOR ] { ALL PRIVILEGES | <privilege list> } ON <object type> <object name> FROM { PUBLIC | <authorization identifier list> [ GRANTED BY { CURRENT_USER | CURRENT_ROLE } ] { RESTRICT | CASCADE } You probably recognize many of the syntax elements from the GRANT statement or from other statements. In fact, the only new component, other than the REVOKE keyword, is the GRANT OPTION FOR clause. Let s take a look at that one first, since it s at the beginning of the REVOKE statement. This clause applies only when the WITH GRANT OPTION clause was used in the GRANT statement. If a privilege was granted with this clause, you can use the GRANT OPTION FOR clause to remove that particular permission. If you do use it, the privileges are preserved, but the user can no longer grant those privileges to other users. However, very few RDBMS products support this clause. Forgetting the GRANT OPTION FOR clause for a moment, let s look at the REVOKE clause itself, which is used to revoke either all privileges on an object (ALL PRIVILEGES) or only the defined privileges (<privilege list>). Both of these options have the same meaning they did in the GRANT statement; you can either use ALL PRIVILEGES or you can list each privilege separated by a comma. The ON clause and GRANTED BY clause in the REVOKE statement are exactly the same as the ON clause and GRANTED BY clause in the GRANT statement. For the ON clause, you must specify values for the <object type> placeholder and the <object name> placeholder; however, if the <object type> value is TABLE, then you can leave that off (and as before you must omit it in Oracle and SQL Server). As for the GRANTED BY clause, assuming your RDBMS supports it (most do not), you can choose one of two options (CURRENT_USER or CURRENT_ROLE).
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