Assuming an order have more that 1 product, you need to tell which product or row in the order you want to update, otherwise all products of order are updated.
This SQL code updates only 1 row.
You need to check if this code is executed more than once.
One way to do it is to add a textbox and append some text each time the code is executed. include the OrderID.
Another way is to use the debugger to see what is doing your code.
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Your code do not behave the way you expect, and you don't understand why !
There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[
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Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[
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Basic Debugging with Visual Studio 2010 - YouTube[
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Debugging C# Code in Visual Studio - YouTube[
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The debugger is here to only show you what your code is doing and your task is to compare with what it should do.