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Reporting duplicate names with non-null addresses

I'm trying to run a report from the database on a list of contacts. The problem is that about 50 names are duplicate...

records because the contact has a primary address and a null address. However, some names (that are not duplicates) have a null address and must be displayed in the report. How can I pull in all contacts with a primary address, and the contacts with a null address, and not have duplicate contacts in my report? Thanks.

If you only wanted distinct names, it would be easy --

     select distinct contactname
       from contacts

But it's trickier than that, because presumably you want addresses on your report as well.

Break it down into two sets. Select all the contacts with non-null addresses. Then select all the contacts with null addresses that don't also have a non-null address --

     select contactname, contactaddress
       from contacts
      where contactaddress is not null
  union all
     select contactname, contactaddress
       from contacts XX
      where contactaddress is null
        and not exists
            ( select * from contacts
               where contactname = XX.contactname
                 and contractaddress is not null )

Use UNION ALL instead of UNION, to avoid the sort to detect duplicate rows. There cannot be any duplicate rows in the combined result set from the two subqueries in the union, because the partial result set from the first subquery has all non-null addresses, whereas the partial result set from the second subquery has all null addresses. The detection of duplicates in a UNION compares all columns of pairs of rows.

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This was last published in November 2001

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