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Subsequent versions of the standard expanded the basic SQL1 privileges in several dimensions. New capabilities were added to the SQL1 SELECT, INSERT, and UPDATE privileges. A new REFERENCES privilege was added that restricts a user s ability to create a reference to a table from a foreign key in another table. Also, a new USAGE privilege was added that controls access to the new SQL database structures of domains, character sets, collation sequences, and translations. The SQL extensions to the SELECT, 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 newer SQL 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 NULL values. 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 SQL REFERENCES privilege deals with a more subtle SQL security issue posed by 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 he has discovered a valid employee number; those that fail represent invalid employee numbers.
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Even more serious problems can be created by a new table defined with a check constraint on a column. For example, suppose the employee tries to execute this CREATE TABLE statement:
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CREATE TABLE XYZ (TRYIT DECIMAL(9,2), CHECK ((SELECT QUOTA FROM SALESREPS WHERE TITLE = 'VP Sales') BETWEEN 400000 AND 500000));
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Because of the column constraint linked to a value from the SALESREPS table, if this statement succeeds, it means the VP of sales has a quota in the specified range. If it doesn t, the employee can keep trying similar CREATE TABLE statements until he has determined the appropriate quota. Note, however, that very few SQL implementations support check constraints that reference other tables. As of this writing, MySQL offers such support, but DB2, SQL Server, and Oracle do not. To eliminate this backdoor access to data, the SQL standard specifies a new REFERENCES privilege. Like the SELECT, INSERT, and UPDATE privileges, the REFERENCES privilege can be granted for specific columns of a table. Only if a user has the REFERENCES privilege for a column is he or she allowed to create a new table that refers to that existing column in any way (for example, as the target of a foreign key reference, or in a check constraint, as in the previous examples). In databases that don t yet implement the REFERENCES privilege but do support foreign keys or check constraints, the SELECT privilege is sometimes used for this purpose. Finally, the SQL standard specifies the USAGE privilege to control access to domains (sets of legal column values), user-defined character sets, collating sequences, and translations. The USAGE privilege is a simple on/off switch that either allows or disallows the use of these database objects, by name, for individual user-ids. For example, with the USAGE privilege on a domain, you can define a new table with a column whose data type is defined as that domain. Without the privilege, you cannot create such a column definition. These privileges are directed mostly toward simplifying administration of large commercial databases that are used and modified by many different development teams. They typically do not present the same kinds of security issues as the table and column access privileges.
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