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Tracking Machines and Counters

Subject: Problem to calculate difference between data in two rows inside one table

Question: What i need is to make database for tracking machine with two counters. 
One counter represent "in" other "out". 
Every machine have some location. 
Once per month we are reading counters from that machines. 
That reading we are comparing with last one and we are getting results. 
For example: locationID=2 have machineID1, machineID2, machineID3 from 
last month machines have some IN and OUT values. 
that values from last month I need to compare with IN-OUT from this months. 

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