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Examples of the first three types of constraints have previously appeared in earlier sections of this chapter. An assertion is specified using the SQL2 CREATE ASSERTION statement. Here is an assertion that might be useful in the demo database: Insure that an office's quota target does not exceed the sum of the quotas for its salespeople: CREATE ASSERTION quota_valid CHECK ((OFFICES.QUOTA <= SUM(SALESREPS.QUOTA)) AND (SALESREPS.REP_OFFICE = OFFICES.OFFICE)) Because it is an object in the database (like a table or a column), the assertion must be given a name (in this case, it's "quota_valid"). The name is used in error messages produced by the DBMS when the assertion is violated. The assertion causing an error may be obvious in a small demo database, but in a large database that might contain dozens or hundreds of assertions, it's critical to know which of the assertions was violated.
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Here is another example of assertion that might be useful in the sample database: Insure that the total of the orders for any customer does not exceed their credit limit: CREATE ASSERTION credit_orders CHECK (CUSTOMER.CREDIT_LIMIT <= SELECT SUM(ORDERS.AMOUNT) FROM ORDERS WHERE ORDERS.CUST = CUSTOMER.CUST_NUM) As these examples show, a SQL2 assertion is defined by a search condition, which is enclosed in parentheses and follows the keyword CHECK. Every time an attempt is made to modify the contents of the database, through an INSERT or UPDATE or DELETE statement, the search condition is checked against the (proposed) modified database contents. If the search condition remains TRUE, the modification is allowed. If the search condition would become untrue, the DBMS does not carry out the proposed modification, and an error code is returned, indicating an assertion violation. In theory, assertions could cause a very large amount of database processing overhead as they are checked for each statement that might modify the database. In practice, the DBMS will analyze the assertion and determine which tables and columns it involves. Only changes that involve those particular tables or columns will actually trigger the search condition. Nonetheless, assertions should be defined with great care to insure that they impose a reasonable amount of overhead for the benefit they provide.
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The types of constraints that can be specified in SQL2, and the role played by each, can be summarized as follows: The NOT NULL constraint can appear only as a column constraint. It prevents the column from being assigned a NULL value. A PRIMARY KEY constraint can appear as a column constraint or a table constraint. If the primary key consists of a single column, the column constraint may be more convenient. If it consists of multiple columns, it should be specified as a table constraint. A UNIQUE constraint can appear as a column constraint or a table constraint. If the unique values restriction is being enforced for a single column only, the column constraint is the easiest way to specify it. If the unique values restriction applies to a set of two or more columns (that is, the combination of values for those columns must be unique for all rows in the table), then the table constraint form should be used. A referential (FOREIGN KEY) constraint can appear as a column constraint or a table constraint. If the foreign key consists of a single column, the column constraint may be more convenient. If it consists of multiple columns, it should be specified as a table constraint. If a table has many foreign key relationships to other tables, it may be most convenient to gather all of its foreign key constraints together at one place in the table definition, rather than having them scattered throughout the column definitions. A CHECK constraint can appear as a column constraint or a table constraint. It is also the only kind of constraint that forms part of the definition of a domain or an assertion. The check constraint is specified as a search condition, like the search condition that appears in the WHERE clause of a database query. The constraint is satisfied if the search condition has a TRUE value.
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Each individual constraint within a database (no matter what its type) may be assigned a constraint name to uniquely identify it from the other constraints. It's probably not necessary to assign constraint names in a simple database where each constraint is clearly associated with a single table, column, or domain, and where there is little potential for confusion. In a more complex database involving multiple constraints on a single table or column, it can be very useful to be able to identify the individual constraints by name (especially when errors start to occur!). Note that the check constraint in an assertion must have a constraint name; this name effectively becomes the name of the assertion containing the constraint.
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