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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 the next chapter.) 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: Insure that an office's quota target is exactly equal to the sum of the quotas for its salespeople. CREATE ASSERTION quota_totals CHECK ((OFFICES.QUOTA = SUM(SALESREPS.QUOTA)) AND (SALESREPS.REP_OFFICE = OFFICES.OFFICE)) 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. The SQL2 deferred constraint mechanism provides for this capability, and much more. Each individual constraint (of all types) within the database can be identified as either DEFERRABLE or NOT DEFERRABLE when it is first created or defined:
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A DEFERRABLE constraint is one whose checking can be deferred to the end of a transaction. The assertion in the previous example is one that should be deferrable. When updating quotas or adding new salespeople to the database, you certainly want to be able to defer constraint checking, as the example showed. A NOT DEFERRABLE constraint is one whose checking cannot be deferred. A primary key constraint, a uniqueness constraint, and many column check constraints would usually fall into this category. These data integrity checks typically don't depend on other database interactions. They can be, and should be, checked after every SQL statement that tries to modify the database. Because it provides the most stringent integrity checking, NOT DEFERRABLE is the default. You must explicitly declare a constraint to be DEFERRABLE if you want to defer its operation. Note also that these constraint attributes only define the deferrability of a constraint that is, whether or not its operation can be deferred. The constraint definition may also specify the initial state of the constraint: An INITIALLY IMMEDIATE constraint is one that "starts out" as an immediate constraint. That is, it will be checked immediately for each SQL statement. An INITIALLY DEFERRED constraint is one that "starts out" as a deferred constraint. That is, its checking will be deferred until the end of a transaction. Of course, this option cannot be specified if the constraint is defined as NOT DEFERRABLE. The constraint is put into the specified initial state when it is first created. It is also reset into this initial state at the beginning of each transaction. Because it provides the most stringent integrity checking, INITIALLY IMMEDIATE is the default. You must explicitly declare a constraint to be INITIALLY DEFERRED if you want it to automatically start out each transaction in a deferred state. SQL2 adds one more mechanism to control the immediate or deferred processing of constraints. You can dynamically change the processing of a constraint during database operation using the SET CONSTRAINTS statement. For example, suppose the sample database contains this assertion: CREATE ASSERTION quota_totals CHECK ((OFFICES.QUOTA = SUM(SALESREPS.QUOTA)) AND (SALESREPS.REP_OFFICE = OFFICES.OFFICE)) DEFERRABLE INITIALLY IMMEDIATE The initially immediate checking causes the constraint to be processed, statement by statement, for all "normal" database processing. For the "special" transaction that adds a new salesperson to the database, however, you will need to temporarily defer constraint processing. This sequence of statements accomplishes the goal: SET CONSTRAINTS quota_totals DEFERRED INSERT INTO SALESREPS (EMPL_NUM, NAME, REP_OFFICE, HIRE_DATE, QUOTA, SALES) VALUES (:num, :name, :office_num, :date, :amount, 0) UPDATE OFFICES SET TARGET = TARGET + :amount WHERE (OFFICE = :office_num) COMMIT After the COMMIT statement ends the transaction, the quota_totals constraint is reset
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back into IMMEDIATE mode because of the INITIALLY IMMEDIATE specification. If there were more work to be done after the UPDATE statement before the end of the transaction, you could manually set the constraint back into IMMEDIATE mode using this statement: SET CONSTRAINTS quota_totals IMMEDIATE You can set the same mode for several different constraints by including the constraint names in a comma-separated list: SET CONSTRAINTS quota_totals, rep_totals IMMEDIATE Finally, you can set the processing mode for all constraints with a single statement: SET CONSTRAINTS ALL DEFERRED The SQL2 capabilities for deferred constraint checking form a very comprehensive facility for managing the integrity of a database. As with many SQL2 capabilities, individual pieces of the SQL2 capability were taken from existing SQL implementations, and individual pieces have found their way into other implementations since the publication of the standard. IBM's DB2, for example, includes deferred constraint checking capability and supports SQL2-style deferrability options. Its SET CONSTRAINTS statement, however, differs from the SQL2 standard. It operates on individual tables in the database, turning on and off the deferral of constraint checking associated with the table contents.
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