Update record if matching values in same table - sql

Question

I have a MS Access database with a list of transactions. I am trying to update a "Match" field on both records where they have some of the same values in fields (Document Number, Voucher Number, Subhead) but opposite amounts. It also needs to avoid duplicates.
Document Number Amount ABS Match
N6809561990112 438.48 438.48
N6809561990112 438.48 438.48
N6809561990112 -438.48 438.48
What I the end result after the SQL should look like
Document Number Amount ABS Match
N6809561990112 438.48 438.48 Y
N6809561990112 438.48 438.48
N6809561990112 -438.48 438.48 Y
The table name is "tblUnmatched"
I tried the following but it updated every record in the table to "Y"
strSql = "Update tblUnmatched SET match = 'Y' WHERE EXISTS(select * " & _
"from tblUnmatched t1 " & _
"inner join tblUnmatched t2 on " & _
"t1.[DOCUMENT NUMBER] = t2.[DOCUMENT NUMBER]" & _
"where t1.ABS = t2.ABS AND t1.AMOUNT <> t2.AMOUNT AND t1.SUBH = t2.SUBH)"
DoCmd.RunSQL strSql
I also tried this but it couldn't handle the duplicate problem.
strSql = "Update tblUnmatched SET match = 'Y' WHERE [DOCUMENT NUMBER] IN(select t1.[DOCUMENT NUMBER] " & _
"from tblUnmatched t1 " & _
"inner join tblUnmatched t2 on " & _
"t1.[DOCUMENT NUMBER] = t2.[DOCUMENT NUMBER]" & _
"where t1.ABS = t2.ABS AND t1.AMOUNT <> t2.AMOUNT AND t1.SUBH = t2.SUBH)"
DoCmd.RunSQL strSql

Answer 1

Your task is impractical in Access SQL without a primary key field. Although there is no such key in the data source you import, that does not prohibit you from adding one in Access.
Add an autonumber primary key, id, to tblUnmatched. Then for each new batch of incoming data:
import into a scratch table, tblScratch
DELETE FROM tblUnmatched
append the rows from tblScratch into tblUnmatched
(The process could be cleaner if you can use SELECT FROM <your data source> to append directly to tblUnmatched, instead of first importing to tblScratch.)
Save the following SELECT statement as qryFindMatches:
SELECT
sub.[Document Number],
sub.Amount,
sub.MinOfid,
DCount(
"*",
"tblUnmatched",
"[Document Number]='" & [Document Number]
& "' AND Amount = -1 * " & [Amount]
) AS match_count
FROM
(
SELECT [Document Number], Amount, Min(id) AS MinOfid
FROM tblUnmatched
GROUP BY [Document Number], Amount
) AS sub;
Then the UPDATE you want can be fairly easy to create. Beware the performance may not be blazing fast; hopefully you can accommodate it once a month.
UPDATE tblUnmatched
SET [Match] = True
WHERE id In
(
SELECT MinOfId
FROM qryFindMatches
WHERE match_count > 0
);
I added an additional row to your sample data and tested with Access 2007. I think this is what you want ...
id Document Number Amount Match
1 N6809561990112 $438.48 True
2 N6809561990112 $438.48 False
3 N6809561990112 ($438.48) True
4 N6809561990112 $5.00 False