Access delete all except max value - sql

Question

I am trying to prune an Access table (contam) full of duplicates and concatenated values.
The table looks like this
FederalSiteIdentifier Contam
001 1
001 1, 2
001 1, 2, 3
001 1, 2, 3, 4
002 1
003 1
003 1, 2
003 1, 2, 3
I only want to keep the last - longest - entry for each ID, but cannot figure out the proper way to do this in Access SQL.
After doing some reading I tried this simple code:
SELECT FederalSiteIdentifier, Max(Contam) as MaxCont
FROM contam
ORDER BY FederalSiteIdentifier
which produces an error.
Can anyone help?

Answer 1

I think you want the maximum length of the Contam values for each FederalSiteIdentifier. You will need a GROUP BY clause.
SELECT FederalSiteIdentifier, Max(Len(Contam)) as MaxCont
FROM contam
GROUP BY FederalSiteIdentifier
If that query identifies the rows you wish to keep, save it as qryRows2Keep, then try this DELETE query:
DELECT FROM contam
WHERE
Len(Contam) < DLookup(
"MaxCont",
"qryRows2Keep",
"FederalSiteIdentifier = '" & FederalSiteIdentifier & "'")
I assumed FederalSiteIdentifier is text data type. If it is numeric, discard the single quotes from the DLookup expression.
Please make sure you have backed up your data before attempting this untested suggestion. :-)

Answer 2

MAX is an aggregate function, you can only do these if you use a GROUP BY statement. Access is also very peculiar in the way it can delete records. You can't left join and delete so you have to identify the records you don't want and then delete them.
Create a new column so we can choose the records to keep.
ALTER TABLE contam ADD Keep BIT NOT NULL DEFAULT 0;
Now identify the records and update the table
UPDATE c
SET keep = 1
FROM contam AS c
INNER JOIN (
SELECT FederalSiteIdentifier, Max(Len(Contam)) as MaxCont
FROM contam
GROUP BY FederalSiteIdentifier
ORDER BY FederalSiteIdentifier
) AS maxc
ON c.FederalSiteIdentifier = maxc.FederalSiteIdentifier
AND Len(c.Contam) = maxc.MaxCont;
Note the GROUP BY line...that's what you missed that was giving you an error..
Finally perform the delete
DELETE FROM contam
WHERE keep = 0;
You can now remove the extra column
ALTER TABLE contam DROP COLUMN Keep;
Long winded but there you go.