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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 ensure 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: I NOT NULL constraint. The NOT NULL constraint can appear only as a column constraint. It prevents the column from being assigned a NULL value. I PRIMARY KEY constraint. 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. I UNIQUE constraint. A UNIQUE constraint can appear as a column constraint or a table constraint. If the unique values restriction is being enforced only for a single column, 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. I Referential (FOREIGN KEY) constraint. 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. I CHECK constraint. 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. 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
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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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In their simplest form, the various constraints that are specified within a database are checked every time an attempt is made to change the database contents that is, during the execution of every attempted INSERT, UPDATE, or DELETE statement. For database systems claiming only Intermediate-Level or Entry-Level conformance to the SQL2 standard, this is the only mode of operation allowed for database constraints. The Full-Level SQL2 standard specifies an additional capability for deferred constraint checking. When constraint checking is deferred, the constraints are not checked for each individual SQL statement. Instead, constraint checking is held in abeyance until the end of a SQL transaction. (Transaction processing and the associated SQL statements are described in detail in 12.) When the completion of the transaction is signaled by the SQL COMMIT statement, the DBMS checks the deferred constraints. If all of the constraints are satisfied, then the COMMIT statement can proceed, and the transaction can complete normally. At this point, any changes made to the database during the transaction become permanent. If, however, one or more of the constraints would be violated by the proposed transaction, then the COMMIT statement fails, and the transaction is rolled back that is, all of the proposed changes to the database are reversed, and the database goes back to its state before the transaction began. Deferred constraint checking can be very important when several updates to a database must all be made at once to keep the database in a consistent state. For example, suppose the demo database contained this assertion: Ensure that an office s quota target is exactly equal to the sum of the quotas for its salespeople.
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CREATE ASSERTION quota_totals CHECK ((OFFICES.QUOTA = SUM(SALESREPS.QUOTA)) AND (SALESREPS.REP_OFFICE = OFFICES.OFFICE))
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Without the deferred constraint checking, this constraint would effectively prevent you from ever adding a salesperson to the database. Why Because to keep the office quota and the salespersons quotas in the right relationship, you must both add a new salesperson row with the appropriate quota (using an INSERT statement) and increase the quota for the appropriate office by the same amount (using an UPDATE statement). If you try to perform the INSERT statement on the SALESREPS table first, the OFFICES table will not yet have been updated, the assertion will not be TRUE, and the statement will fail. Similarly, if you try to perform the UPDATE statement on the OFFICES table first, the SALESREPS table will not yet have been updated, the assertion will not be TRUE, and the statement will fail. The only solution to this dilemma is to defer constraint checking until both statements have completed, and then check to make sure that both operations, taken together, have left the database in a valid state.
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