Question

I created a table with the following fields:

Record:
Id                 int Primary Key, Auto Increment
ForeignId          int
IsDuplicateRecord bit NULL

Then I inserted some data:

INSERT INTO Record (ForeignId)
VALUES (5), (5), (1), (2), (3)

After that, I ran the following update statement, (found at http://archive.msdn.microsoft.com/SQLExamples/Wiki/View.aspx?title=DuplicateRows ):

UPDATE Record
SET IsDuplicateRecord = 1
WHERE Id IN (
    SELECT MAX(Id)
    FROM Record
    GROUP BY ForeignId
    HAVING COUNT(*) > 1
)

So far so good, the query affected one row, and the table now looks like this:

Id ForeignId IsDuplicateRecord
0  5         NULL
1  5         1
2  1         NULL
3  2         NULL
4  3         NULL

I was happy, because for a moment I thought everything was going to be just fine. But then a suspicion as dark as the clouds outside crossed my mind:
Dreadingly, I typed

INSERT INTO Record (ForeignId)
VALUES (1), (1)

and ran the above query again, which this time yielded:

Id  ForeignId  IsDuplicateRecord
0   0          NULL
1   5          1
2   1          NULL
3   2          NULL
4   3          NULL
5   1          NULL
6   1          1

So I figured I’d head over to StackOverflow, and see who could explain to me why the IsDuplicatedRecord field in row with ID 5 wasn’t updated to 1? Are you the one?

Answer 1

Because the SQL you ran only marks the last of the duplicates as duplicates. Try this instead:

UPDATE Record
SET IsDuplicateRecord = 1
WHERE Id NOT IN (
    SELECT MIN(Id)
    FROM Record
    GROUP BY ForeignId
)

This marks second and subsequent occurences of each ForeignId as duplicates as I think you require.