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Inside Microsoft SQL Server 2008: T-SQL Programming
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table for each row, run sp_executesql to return the result of the computation through an output parameter to a local variable defined in the trigger, and use an UPDATE statement to store the result in a result column in the table . Taking this approach would make the table resemble a Microsoft Office Excel spreadsheet . Here I ll represent the problem in more generic terms . To demonstrate the technique, first run the following code to create the Computations table:
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USE tempdb; IF OBJECT_ID('dbo.Computations', 'U') IS NOT NULL DROP TABLE dbo.Computations; CREATE TABLE dbo.Computations ( keycol INT NOT NULL IDENTITY PRIMARY KEY, arg1 INT NULL, arg2 INT NULL, arg3 INT NULL, computation VARCHAR(4000) NOT NULL, result INT NULL, CONSTRAINT CHK_Computations_SQL_Injection CHECK (REPLACE(computation,'@arg','') NOT LIKE '%[^0-9.+/* -]%') );
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The columns arg1, arg2, and arg3 will hold the input arguments for the computation . The computation column will hold T-SQL expressions that refer to the inputs using an @ symbol in front of each argument (for example, @arg1 would stand for the value in arg1) . Examples of expressions are as follows: @arg1 + @arg2 + @arg3 , @arg1 * @arg2 @arg3 , 2. * @arg2 / @arg1 , or any other valid T-SQL expression that yields a scalar value as a result . A CHECK constraint is defined on the computation column to protect against SQL injection attempts . The constraint allows only arguments (@arg), digits, dots, and basic arithmetic operations; you may want to revise the constraint based on your needs, but bear in mind that the more you relax the constraint, the greater is the risk that SQL injection attempts will succeed . The trigger should evaluate the expression from each modified row and store the result value in the result column . Run the following code to create the trg_Computations_iu_calc_result trigger:
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CREATE TRIGGER trg_Computations_iu_calc_result ON dbo.Computations FOR INSERT, UPDATE AS DECLARE @rc AS INT = (SELECT COUNT(*) FROM (SELECT TOP (2) * FROM inserted) AS D); -- If no rows affected, return IF @rc = 0 RETURN; -- If none of the columns: arg1, arg2, arg3, computation -- were updated, return
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IF COLUMNS_UPDATED() & 30 /* 00011110 binary */ = 0 RETURN; -- Not allowed to update result IF EXISTS(SELECT * FROM inserted) AND EXISTS(SELECT * FROM deleted) AND UPDATE(result) BEGIN RAISERROR('Not allowed to update result.', 16, 1); ROLLBACK; RETURN; END DECLARE @key @in_arg1 @in_arg2 @in_arg3 @out_result @comp @params
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INT, INT, INT, INT, INT, NVARCHAR(4000), NVARCHAR(100);
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keycol arg1 arg2 arg3 result of computation computation parameter's list for sp_executesql
-- If only one row was affected, don't use a cursor IF @rc = 1 BEGIN -- Grab values from inserted SELECT @key = keycol, @in_arg1 = arg1, @in_arg2 = arg2, @in_arg3 = arg3, @comp = N'SET @result = ' + computation FROM inserted; -- Generate a string with the in/out parameters SET @params = N'@result INT output, @arg1 INT, @arg2 INT, @arg3 INT'; -- Calculate computation and store the result in @out_result EXEC sp_executesql @comp, @params, @result = @out_result OUTPUT, @arg1 = @in_arg1, @arg2 = @in_arg2, @arg3 = @in_arg3; -- Update the result column in the row with the current key UPDATE dbo.Computations SET result = @out_result WHERE keycol = @key; END -- If only multiple rows were affected, use a cursor ELSE BEGIN -- Loop through all keys in inserted DECLARE CInserted CURSOR FAST_FORWARD FOR SELECT keycol, arg1, arg2, arg3, N'SET @result = ' + computation FROM inserted; OPEN CInserted;
Inside Microsoft SQL Server 2008: T-SQL Programming
-- Get first row from inserted FETCH NEXT FROM CInserted INTO @key, @in_arg1, @in_arg2, @in_arg3, @comp ; WHILE @@fetch_status = 0 BEGIN -- Generate a string with the in/out parameters SET @params = N'@result INT output, @arg1 INT, @arg2 INT, @arg3 INT'; -- Calculate computation and store the result in @out_result EXEC sp_executesql @comp, @params, @result = @out_result OUTPUT, @arg1 = @in_arg1, @arg2 = @in_arg2, @arg3 = @in_arg3; -- Update the result column in the row with the current key UPDATE dbo.Computations SET result = @out_result WHERE keycol = @key; -- Get next row from inserted FETCH NEXT FROM CInserted INTO @key, @in_arg1, @in_arg2, @in_arg3, @comp; END CLOSE CInserted; DEALLOCATE CInserted; END GO
The trigger first evaluates the number of rows that were affected by the firing statement (INSERT, UPDATE, or MERGE) . If zero rows were modified, the trigger simply terminates . It has nothing to do in such a case . The trigger then checks whether one of the four relevant columns (arg1, arg2, arg3, computation) was modified, using the COLUMNS_UPDATED() function . I described this function in more detail in 4, Triggers . This function returns a bitmap with a representative bit for each column . For an UPDATE statement, the bit is turned on if the corresponding column was specified in the SET clause, and it s turned off if the corresponding column wasn t specified in the SET clause . For an INSERT statement, all column bits are turned on . If none of the relevant columns were modified, the trigger simply terminates . It has no reason to reevaluate the computation if neither the inputs nor the computation changed . If the statement that fired the trigger was an UPDATE statement (or a MERGE statement containing an UPDATE action), and the column result was modified, the trigger generates an error message and rolls back the update . The trigger defines local variables to host the input arguments, the computation, and the result value . Each row must be handled separately, so a cursor is needed if there is more than one row .
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