Identifying the Number of Affected Rows in VS .NET

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Identifying the Number of Affected Rows
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Remember that triggers are fired once per statement, not once per affected row . This means that a trigger will fire once whether zero, one, or more than one rows were affected by the triggering statement . In most cases, if a trigger fired, but zero rows were affected, you don t want the trigger to react . If one row was affected, you can typically apply simple logic and query inserted or deleted directly, knowing that it contains only one row . For example, you can safely use an assignment SELECT like the following one to grab data from that row:
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SELECT @var1 = col1, @var2 = col2, FROM inserted;
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However, you will face logical issues with such a statement if zero or multiple rows were affected . With zero affected rows, such a statement does not perform any assignment at all, so the variables will retain the values they had earlier . With multiple affected rows, this statement will perform multiple assignments one for each row . However, the variables will have the values assigned to them in the last assignment performed . If you just assume that your triggers will fire only for single-row modifications, but they don t, you ll end up with logical bugs . Suppose that you use a SET command to assign a value from inserted or deleted to a variable:
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SET @var1 = (SELECT col1 FROM inserted);
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If one row was affected, this assignment will work perfectly well . With zero affected rows, the subquery will return NULL . With multiple affected rows, this code will break, generating an error . You need to take these things into consideration when programming triggers so that you can avoid bugs and realize good performance . You can determine a course of action based on the number of rows affected by the triggering statement . Later I ll explain how you can efficiently tell how many rows were affected .
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With zero affected rows, you typically just want to return from the trigger there s no point in wasting resources if you need not do anything . If more than zero rows were affected, the course of action you should take depends on the type of activity you want to issue from the trigger . If you can achieve the activity by using a set-based join between inserted or deleted and the base table, you can apply the same code for both single and multiple affected rows . For example, suppose that you re writing an INSERT trigger for a table called T1 that is supposed to modify an attribute called col1 in the new rows . You can achieve this by using an UPDATE statement that is based on a join between T1 and inserted, like so:
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UPDATE T1 SET col1 = <expression> FROM T1 JOIN inserted AS I ON T1.keycol = I.keycol;
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In SQL Server 2008 you can use the standard MERGE statement instead of the above nonstandard UPDATE statement, like so:
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MERGE INTO T1 USING inserted AS I ON T1.keycol = I.keycol WHEN MATCHED THEN UPDATE SET col1 = <expression>;
Either way, the purpose of the join is just to filter the new rows added to T1 . However, suppose that you need to deal with each new row individually using iterative logic . You need to retrieve values from each row individually and take some action . Remember that if only one row was affected, you can safely use an assignment SELECT to grab values from the row in inserted or deleted:
SELECT @var1 = col1, @var2 = col2, FROM inserted;
So you can check the number of affected rows, and if it is one, use such logic . Otherwise, you need to apply iterative logic . Remember that inserted and deleted are not indexed . You can copy the data from the special table into a temporary table, index it, and then iterate through the rows in the temporary table using a loop . A more efficient solution is to use a cursor based on a query against inserted or deleted . With the latter approach you do not need to copy the data to a temporary table and index it . To demonstrate the flow handling based on the number of affected rows, first run the following code to create the table T1:
SET NOCOUNT ON; USE tempdb; IF OBJECT_ID('dbo.T1', 'U') IS NOT NULL DROP TABLE dbo.T1;
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