sql - How to take difference between current data and previous? -


i working on access 2010 table contains date, unit id, meter reading, correction. want take daily reading of meter taking current date , subtracting previous. here example of table have:

inputdate | unitid | meterreading | correction --------- | ------ | ------------ | ---------- 11/1/2016 | u1     | 41476        | 10000 11/1/2016 | u2     | 5991         | 10000 11/1/2016 | u3     | 164205       | 1000000 11/1/2016 | u4     | 11160        | 100000 11/2/2016 | u1     | 41490        | 10000 11/2/2016 | u2     | 5991         | 10000 11/2/2016 | u3     | 164205       | 1000000 11/2/2016 | u4     | 11169.3      | 100000

i've tried number of different ways. way got work make query split table each unit (e.g. u1, u2, etc.). query result , calculation: 

( ([meterreading]+[correction]) -  (dlookup("meterreading", "tbla", "[tbla].[inputdate] = #" &  dateadd("d",-1,[tbla].[inputdate]) & "#")+[correction]) )

then query group data date.

my question if there more intuitive way this? i've looked @ ways try doing sql queries, nested queries, , grouping unit_id, calculation off. think dlookup not looking @ right unit when doing calculation since there multiple units on same day. also, "your query not include specified expression" when try group inputdate , unitid. i'm new sql language appreciated.

sample output be:

inputdate | unitid | meterruntime --------- | ------ | ------------ 11/2/2016 | u1     | 14 11/2/2016 | u2     | 0 11/2/2016 | u3     | 0 11/2/2016 | u4     | 9.3

if i'm understanding correctly following should give need.

select tbla.inputdate, tbla.unitid, ([tbla].[meterreading]+[tbla].[correction])- ([tbla_1].[meterreading]+[tbla_1].[correction]) meterruntime  tbla inner join tbla tbla_1  on (tbla.inputdate-1 = tbla_1.inputdate) , (tbla.unitid = tbla_1.unitid);

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