посоветовал бы для начала почитать по доменным агрегатным функциям - например в Хелп заглянуть

DFirst, DLast Functions

You can use the DFirst and DLast functions to return a random record from a particular field in a table or query when you simply need any value from that field. Use the DFirst and DLast functions in a macro, module, query expression, or calculated control on a form or report.

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Remarks

Note If you want to return the first or last record in a set of records (a domain), you should create a query sorted as either ascending or descending and set the TopValues property to 1. For more information, see the TopValues property topic.

From Visual Basic, you can also create an ADO Recordset object and use the MoveFirst or MoveLast method to return the first or last record in a set of records.

если нужно возвратить минимальное значение из таблицы подойдет Dmin("[PostID]","[post]")

PS:

First <> Min AND Last <> Max

А в 97 справочка поудобней была....