Unanswered: Subtract previous record from current one
I have a table that holds tool readings for certain dates. The readings determine usage and are integer values. Tool readings are noted every few days and inputed into the table all at once. The table is structured as such:
date_measured; tool_1; tool_2; tool_3
7/15/02; 7794; 865; 22304
7/19/02; 7836; 910; 22376
7/22/02; 7887; 967; 22414
and so on...
I have a query that sorts the table in descending order by date. I need to determine the difference in dates and tool readings in adjacent records. For instance: 7/22/02 - 7/19/02 = 3 days; 7887 - 7836 = 51 (tool_1). I can use this data to determine average daily usage (51 divided by 3 days = 17 per day).
How can I subtract adjacent records in a query? Please help.