#### Topic: How to deduce units, which now in operation?

Hello, I the beginning user. There was a problem, it is necessary to learn what units now are in operation. There are tables arrival of units, closing of units. The unit has a manager No. I think so, the first record from Arrival undertakes, the manager No is checked with . from Closing if coincidence is not present it is deduced if there is a coincidence then the unit is written off, it is not necessary to deduce. Somehow so. But how to implement I can not understand. In advance thanks