在 SQL 中，在另一列中查找具有唯一值的列中的重复项

Question

So I have a table of aliases linked to record ids. I need to find duplicate aliases with unique record ids. To explain better:

ID    Alias     Record ID
1     000123    4
2     000123    4
3     000234    4
4     000123    6
5     000345    6
6     000345    7

The result of a query on this table should be something to the effect of

000123    4    6
000345    6    7

Indicating that both record 4 and 6 have an alias of 000123 and both record 6 and 7 have an alias of 000345.

I was looking into using GROUP BY but if I group by alias then I can't select record id and if I group by both alias and record id it will only return the first two rows in this example where both columns are duplicates. The only solution I've found, and it's a terrible one that crashed my server, is to do two different selects for all the data and then join them

ON [T_1].[ALIAS] = [T_2].[ALIAS] AND NOT [T_1].[RECORD_ID] = [T_2].[RECORD_ID]

Are there any solutions out there that would work better? As in, not crash my server when run on a few hundred thousand records?

Answer 1

It looks as if you have two requirements:

1. Identify all aliases that have more than one record id, and
2. List the record ids for these aliases horizontally.

The first is a lot easier to do than the second. Here's some SQL that ought to get you where you want with the first:

WITH A   -- Get a list of unique combinations of Alias and [Record ID]
AS  (
   SELECT Distinct
          Alias
     ,    [Record ID]
   FROM  T1
)
,   B  -- Get a list of all those Alias values that have more than one [Record ID] associated
AS  (
    SELECT Alias
    FROM   A
    GROUP BY
           Alias
    HAVING COUNT(*) > 1
)
SELECT  A.Alias
    ,   A.[Record ID]
FROM    A
    JOIN B
        ON  A.Alias = B.Alias

Now, as for the second. If you're satisfied with the data in this form:

Alias     Record ID
000123    4
000123    6
000345    6
000345    7

... you can stop there. Otherwise, things get tricky.

The PIVOT command will not necessarily help you, because it's trying to solve a different problem than the one you have.

I am assuming that you can't necessarily predict how many duplicate Record ID values you have per Alias , and thus don't know how many columns you'll need.

If you have only two, then displaying each of them in a column becomes a relatively trivial exercise. If you have more, I'd urge you to consider whether the destination for these records (a report? A web page? Excel?) might be able to do a better job of displaying them horizontally than SQL Server can do in returning them arranged horizontally.

Answer 2

Perhaps what you want is just the min() and max() of RecordId :

select Alias, min(RecordID), max(RecordId)
from yourTable t
group by Alias
having min(RecordId) <> max(RecordId)

You can also count the number of distinct values, using count(distinct) :

select Alias, count(distinct RecordId) as NumRecordIds, min(RecordID), max(RecordId)
from yourTable t
group by Alias
having count(DISTINCT RecordID) > 1;

Answer 3

This will give all repeated values:

select Alias, count(RecordId) as NumRecordIds,  
from yourTable t
group by Alias
having count(RecordId) <> count(distinct RecordId);