Select duplicates/repeating values, ungrouped - mysql

Question

I am trying to return a list of ungrouped values having a max defined number of repeating values. I have a list of values retrieved as such:
select TagDirID from tags where id = '550'
The results would be:
9508
10382
10672
65454
65454
65454
65454
As you can see there are 4 repeats of 65454. I would like to return a list that has a user defined max number of repeats for a TagDirID. For instance, selecting only 3 repeats or less:
9508
10382
10672
65454
65454
65454
All the methods I've found return a grouped list, i would like to maintain the individual items. Is this something that can be done in a query? There is a primary key available, TagID.
edit: what this does is select all the tags for an item, id = '550'. because it's user content sometimes people tag the same things multiple times and i'm trying to cut down the duplicates i show.
edit 2: so, while the accepted answer worked for me i found it was a little too slow for what i needed so i came up with a php solution:
function get_tags($ID = '', $tags_to_keep = 3)
{
// Select all tags.
$query = "select TagDirID, Tag from tags where id = '$ID'";
$tags_result = mysql_query($query);
$num_results = mysql_num_rows($tags_result);
for ($i=0; $i<$num_results; $i++)
{
//Get tag topics
$tags_row = mysql_fetch_array($tags_result);
//build array of items already found with counts
$tags_count = array_count_values($tags_filter);
//if number of tags already found($tags_count/$tags_filter) is less than or equal to tags_to_keep then add to filtered array and return array.
if($tags_count[$tags_row['TagDirID']] <= $tags_to_keep)
{
$tags_filter[$i] = $tags_row['TagDirID'];
$tags[$i] = $tags_row['Tag'];
}
}
return $tags;
}

Answer 1

Select TagID, TagDirID
From (
Select T1.TagID, T1.TagDirID
, (Select Count(*)
From tags As T2
Where T2.TagDirID = T1.TagDirID
And T2.TagID < T1.TagID) As Rnk
Where T1.id = '550'
From tags As T1
) As T
Where T.Rnk < 3
Another way of writing the same query:
Select TagID, TagDirID
From (
Select T1.TagID, T1.TagDirID, Count( T2.TagID ) As Rnk
From tags As T1
Left Join tags As T2
On T2.TagDirID = T1.TagDirID
And T2.TagID < T1.TagID
Where T1.id = '550'
Group By T1.TagID, T1.TagDirID
) As T
Where T.Rnk < 3
The approach here is to mimic a ranking function which would sequence the rows for each grouping of TagDirID. Thus, the inner query in either of the two above solutions should give you something like:
TagID | TagDirID | Rnk
1 | 9508 | 0
2 | 10382 | 0
3 | 10672 | 0
4 | 65454 | 0
5 | 65454 | 1
6 | 65454 | 2
7 | 65454 | 3
With the rows numbered within each grouping, we can now filter down our results so that we only get a maximum number of rows in any given group. The ISO/ANSI solution would be to use the ROW_NUMBER ranking function which isn't yet supported by MySQL.

Answer 2

As the repeated values are all equal, you can query for the number of repetions of distinct items. It would be something like this, for 3 items or less:
SELECT T.tdid, T.cnt
FROM (
SELECT distinct(TagDirID) as tdid,
(SELECT COUNT(*) FROM tags WHERE id = tdid) as cnt
FROM tags
) as T
WHERE T.tdid = '550' AND T.cnt < 3;
So the result would be a little different from what you had (not the duplicate items, but one item and the number of duplications), but I think it will do.