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21-Feb-2019 03:52

Multirow considerations are especially important when the function of a DML trigger is to automatically recalculate summary values from one table and store the results in another for ongoing tallies.

The DML triggers in the following examples are designed to store a running total of a column in another table of the Adventure Works2012 sample database.

Purchase Order Detail FOR INSERT AS IF @@ROWCOUNT = 1 BEGIN UPDATE Purchasing.

USE Adventure Works2012; GO CREATE TRIGGER New PODetail2 ON Purchasing.

Target Table AS tgt INNER JOIN ( VALUES (1, 'a', 'k', 'x', ...), (2, 'b', 'l', 'y', ...), (3, 'c', 'm', 'z', ...) ) AS src (ID, Column1, Column2, Column3, ...) ON tgt.

If you omit the WHERE clause, all records in the table will be updated!

UPDATE summary_data SET current_category = (SELECT category_id FROM products WHERE products.product_id = summary_data.product_id) WHERE EXISTS (SELECT category_id FROM products WHERE products.product_id = summary_data.product_id); If you want to test your skills using the SQL UPDATE statement, try some of our practice exercises.

These exercises allow you to try out your skills with the UPDATE statement.When you write the code for a DML trigger, consider that the statement that causes the trigger to fire can be a single statement that affects multiple rows of data, instead of a single row.This behavior is common for UPDATE and DELETE triggers because these statements frequently affect multiple rows.Purchase Order Detail AFTER INSERT AS UPDATE Purchasing. Purchase Order ID IN (SELECT Purchase Order ID FROM inserted); -- Trigger valid for multirow and single row inserts -- and optimal for single row inserts.

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I've had to figure this out on my own twice now, so I guess it's time to document it. It is possible, and fairly easy, to update multiple rows of a MySQL table at the.… continue reading »

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