CREATE VIEW View1 AS SELECT Table1*, Function1(city) AS Function_return FROM Table1 in Java

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CREATE VIEW View1 AS SELECT Table1*, Function1(city) AS Function_return FROM Table1
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And the CREATE FUNCTION statement would look like this:
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CREATE FUNCTION Function1 (parameter1 CHAR(10)) RETURNS SMALLINT LANGUAGE C PARAMETER STYLE GENERAL NOT DETERMINISTIC RETURN NULL ON NULL INPUT NO SQL EXTERNAL
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Finally, the external function code would look like Listing 10-4 Listing 10-4 External function code for SELECT trigger
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short* __export __cdecl FUNCTION1 (char* c) { static short return_value; HANDLE FileHandle; HANDLE MutexHandle; unsigned long ret; if (lstrcmp(c,"Washington")==0) { MutexHandle=CreateMutex(0,TRUE,"MUTEX"); FileHandle=CreateFile( "T", GENERIC_WRITE, 0, 0, OPEN_ALWAYS, FILE_ATTRIBUTE_NORMAL, 0); SetFilePointer(FileHandle,0,0,FILE_END); WriteFile(FileHandle,"it happened",11,&ret,0);
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CloseHandle(FileHandle); CloseHandle(MutexHandle); return_value=ret; } else return_value=0; return (&return_value); }
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The matter is not simple you must persuade the DBMS that the function is unpredictable and can change the column value Otherwise, the DBMS will optimize the function call and only do it once To persuade the DBMS not to optimize, write the function in a language the DBMS doesn't understand natively, make the column input/output (in fact the column isn't touched), and above all declare the function as NOT DETERMINISTIC A word of caution about NOT DETERMINISTIC functions In real triggers, it is possible to call stored procedures with nondeterministic functions, but there is a peril Our favorite one triggers an action that depends on the time of day, because "for some reason" time of day can't be used in a CHECK clause Well, some people would think that reason is important suppose the database becomes corrupt, and the recovery procedure involves rerunning a log of transactions In such a case, the recovery isn't run at the same time of day as the original transaction and thus a transaction that succeeded before the crash will fail during the recovery Remember that some DBMSs handle functions in select lists when they open the cursor; others handle them when they fetch from the cursor
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The Bottom Line: Triggers
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If speed is your only concern, use declarative constraints instead of triggers Use views with an external function call to write a SELECT trigger Declarative constraints differ from triggers in that constraint conditions guarantee that data always fulfills a specific requirement, while trigger conditions are enforced only when data changes That is, constraints are passive requirements for correctness, while triggers are responses only to specific actions Because trigger checking happens after constraint checking, it's tricky to change the value of a table column within the trigger That could lead to loops (because the change would cause another constraint check) or to subversions (because the change wouldn't go through the same checking that a regular data-change statement would go through) The SQL Standard has special rules that are designed to prevent such tricks Triggers fire on INSERT, UPDATE, or DELETE statements but not on "implied" inserts like the DBMS's nonstandard database LOAD statement and not on "implied" deletes like the DBMS's nonstandard table TRUNCATE statement
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Disabling Constraints
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There are three ways to allay or prevent constraints: defer, disable, drop
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Defer
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The standard way to allay or prevent constraints is to defer the constraint To use this method, you first have to explicitly name the constraints on a table and define them as deferrable, like this:
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CREATE TABLE Table1 ( column1 SMALLINT, CONSTRAINT Constraint1 CHECK ( column1 < 1000) DEFERRABLE INITIALLY DEFERRED)
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A constraint defined as DEFERRABLE allows you to specify when you want the DBMS to check the constraint for violation the choices are after the SQL statement is executed or at transaction end The INITIALLY DEFERRED keywords ensure that the DBMS will defer checking the constraint either until transaction end or until you activate the deferrable constraints in your program with a SET CONSTRAINTS statement, like this:
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SET CONSTRAINTS ALL IMMEDIATE
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Remember that constraint deferring only happens within a transaction as soon as you COMMIT, the DBMS will check for constraint violations With deferrals, though, you can gain some control over the timing of constraint checks, which is especially an advantage if cyclic references are in your table definitions, or if there is an assumption that the transaction might have to be canceled with ROLLBACK anyway Portability At the moment, SQL Standard-compliant deferral is supported only by IBM and Oracle Informix supports the SET CONSTRAINTS statement, but all Informix constraints are deferrable by definition Microsoft supports deferrable constraints, but not with standard SQL syntax
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