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SQL security protections apply to specific objects contained in a database. The SQL1 standard specified two types of security objects tables and views. Thus, each table and view can be individually protected. Access to a table or view can be permitted for certain user-ids and prohibited for other user-ids. The SQL2 standard expanded security protections to include other objects, including domains and user-defined character sets, and added a new type of protection for table or view access.
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Most commercial SQL products support additional security objects. In a SQL Server database, for example, a stored procedure is an important database object. The SQL security scheme determines which users can create and drop stored procedures and which users are allowed to execute them. In IBM s DB2, the physical tablespaces where tables are stored are treated as security objects. The database administrator can give some user-ids permission to create new tables in a particular tablespace and deny that permission to other user-ids. Other SQL implementations support other security objects. However, the underlying SQL security scheme of specific privileges applied to specific objects, granted or revoked through the same SQL statements is almost universally applied.
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The set of actions that a user can carry out against a database object are called the privileges for the object. The SQL1 standard specifies four basic privileges for tables and views: I The SELECT privilege allows you to retrieve data from a table or view. With this privilege, you can specify the table or view in the FROM clause of a SELECT statement or subquery. I The INSERT privilege allows you to insert new rows into a table or view. With this privilege, you can specify the table or view in the INTO clause of an INSERT statement. I The DELETE privilege allows you to delete rows of data from a table or view. With this privilege, you can specify the table or view in the FROM clause of a DELETE statement. I The UPDATE privilege allows you to modify rows of data in a table or view. With this privilege, you can specify the table or view as the target table in an UPDATE statement. The UPDATE privilege can be restricted to specific columns of the table or view, allowing updates to these columns but disallowing updates to any other columns. These four privileges are supported by virtually all commercial SQL products.
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The SQL2 standard expanded the basic SQL1 privileges in several dimensions. It added new capabilities to the SQL1 INSERT and UPDATE privileges. It added a new REFERENCES privilege that restricts a user s ability to create a reference to a table from a foreign key in another table. It also added a new USAGE privilege that controls access to the new SQL2 database structures of domains, character sets, collation sequences, and translations.
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The SQL2 extensions to the INSERT and UPDATE privileges are straightforward. These privileges may now be granted for a specific column or columns within a table, instead of applying to the entire table. The sample database provides a simple example of how this capability can be useful. Suppose you wanted to give your human resources manager the responsibility to insert new employees into the SALESREPS table, once the hiring paperwork is complete. The HR manager should supply the employee number, name, and similar information. But it should be the responsibility of the sales VP to set the QUOTA column for the new employee. Adjustments to the SALES column for existing employees would be similarly restricted. Using the new SQL2 capabilities, you could implement this scheme by giving the HR manager INSERT privileges on the appropriate columns. The other columns (such as SALES and QUOTA) for any newly inserted employees would initially have the NULL value. With the UPDATE privilege on the other columns, the sales VP can then set the appropriate quota. Without the ability to specify these privileges on specific columns, you would have to either relax the restrictions on column access or define extraneous views on the table simply to restrict access. The SQL2 standard does not allow the SELECT privilege to be applied to specific columns like the new INSERT and UPDATE capabilities; it must still be specified for an entire table. Theoretically, this capability isn t really needed, since you can achieve the same effect by defining a view on the table, limiting the view to specific columns, and then defining the appropriate privileges on the view. However, a column-specific SELECT privilege can be a much more straightforward approach. It keeps the structure of the database simpler (fewer view definitions) and concentrates the security scheme more tightly in one place (the GRANT statements). Several major DBMS brands, including Sybase and SQL Server, allow you to specify column-specific SELECT privileges, using the same syntax as for the column-specific UPDATE and INSERT. The SQL2 standard includes a note that this capability is also intended to be considered for future updates of the standard. The new SQL2 REFERENCES privilege deals with a more subtle SQL security issue posed by the SQL2 capabilities of foreign keys and check constraints. Using the sample database as an example, suppose an employee has the ability to create a new table in the database (for example, a table containing new product information) but does not have any access to the employee information in the SALESREPS table. You might assume, given this security scheme, that there is no way for him to determine the employee numbers being used or whether a new employee has been hired. However, this isn t strictly true. The employee could create a new table, with a column that is defined as a foreign key to the SALESREPS table. (Recall that this means the only legal values for this column are primary key values for the SALESREPS table that is, valid employee numbers.) With this new table, the employee can simply try to insert new rows with different values in the foreign key column. The INSERT statements that succeed tell the employee that that he has discovered a valid employee number; those that fail represent invalid employee numbers.
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