SQL will have to actually compute the numerical result before it does anything else, it must do it so that it knows what value it should "do something". Even then, for comparison, you need to read the value from the table.
What I'm trying to say is that if that were the case, it would actually make it less efficient to read the value, compare it with the one you are trying to update, and then decide whether it should perform the update operation. In your case, he must read Qty
before he can determine what he needs to put in field A
, but even then, by the time he compared the values, he could also complete the update and continue with the rest of his busy days :)
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