Re: selecting duplicate records

The world rejoiced as ch(at)rodos(dot)fzk(dot)de (Christoph Haller) wrote:
>> 1. How to select duplicate records only from a single table using a
> select
>> query.
>>
> e.g.
> select sid,count(sid) from location group by sid having count(sid)>1;
>
> Do you get the idea?
> Your request is pretty unspecific, so if this is not what you're asking
> for,
> try again.

The aggregate is likely to perform horrifically badly. Here might
be an option:

Step 1. Find all of the duplicates...

select a.* into temp table sid from some_table a, some_table b
where a.oid < b.oid and
a.field1 = b.field1 and
a.field2 = b.field2 and
a.field3 = b.field3 and
...
a.fieldn = b.fieldn;

Step 2. Look for the matching entries in the source table...

select a.* from some_table a, sid b
where
a.field1 = b.field1 and
a.field2 = b.field2 and
a.field3 = b.field3 and
...
a.fieldn = b.fieldn;

[There's a weakness here; if there are multiple dupes, they may get
picked multiple times in the second query :-(.]
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