display warning when users enter a value greater than the actual value
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can anyone give me some idea how to write a code to display warning when users enter a value greater than the actual value, but it still can enter values ​​for other fields. I use VB.net and access database. I think it is able to use onchange but i still don't have any idea to write the code

here is example 
![enter image description here][1]


  [1]: http://i.stack.imgur.com/xFuSe.png