megaSteve4 megaSteve4 - 19 days ago 10
SQL Question

query that would count and increment the number of duplicate instances of that record

Using Access 2010.

So if I had a table

COL1
A
B
A
C
A


and the run the query I would get the output in COL2 where 'A' is duplicated three times and its COL2 value is in turn incremented.

COL1 | COL2
A | 1
B | 1
A | 2
C | 1
A | 3

Answer

Add a field to your table. Choose AutoNumber as its data type and make it the table's primary key. I named the field ID, so my version of your sample data looks like this ...

ID COL1
 1 A
 2 B
 3 A
 4 C
 5 A

The SELECT statement below returns this result set ...

ID COL1 COL2a COL2b
 1 A        1     1
 2 B        1     1
 3 A        2     2
 4 C        1     1
 5 A        3     3

COL2a and COL2b show 2 methods to achieve the same result. DCount is Access-specific, and required quotes around the m.COL1 text values. The second approach, COL2b, uses a correlated subquery so could work in a different database if you choose. And with that approach, you wouldn't need to bother about quoting text values.

Either approach basically requires the db engine run an additional query for each row of the result set. So, with a huge table, performance will be a concern. Indexing will help there. Add an index on COL1 if there isn't one already. ID already has an index since it's the primary key.

If you can't add a field, and the table doesn't already include another suitable field, then I think you're out of luck. You won't be able to get what you want with an Access query.

SELECT
    m.ID,
    m.COL1,
    DCount(
        "*",
        "MyTable",
        "COL1 = '" & m.COL1 & "' AND ID <= " & m.ID
    ) AS COL2a,
    (
        SELECT Count(*)
        FROM MyTable AS m2
        WHERE m2.COL1 = m.COL1 AND m2.ID <= m.ID
    ) AS COL2b
FROM MyTable2 AS m
ORDER BY m.ID;
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