Tip

Removing duplicate records from a table that has no primary key

There are situations where you may have to rethink queries to tackle complex problems. Here is one of them. We had a table in a SQL Server database called NAMELINK where no primary key was defined. The structure was as follows: 

NameId1         int       4       not null
Category        char      5       nullable
Relation12      char      20      nullable
NameId2         int       4       not null
Relation21      char      21      nullable
Remarks         char      30      nullable
Dependent       smallint  2       not null
While converting to another database, we assumed that NameId1 and NameId2 are composite primary keys. There may be some cases where they aren't unique. Now we wanted a select query to fetch only those records where the combination of NameId1 and NameId2 is unique.

To be more clear, if the values are as follows: 

Nameid1 Nameid2
1       2
2       1
Then the select query should fetch only one combination and not both.

Here is the query. It has been tested on SQL Server 7.0: 

SELECT NAMELINK.* FROM NAMELINK LEFT JOIN
(SELECT nl1.* FROM NAMELINK nl1 INNER JOIN NAMELINK nl2 on
nl1.Nameid2 = nl2.Nameid1 and nl1.Nameid1 = nl2.Nameid2
) MyNameLink ON Namelink.Nameid1 = MyNameLink.Nameid1
and Namelink.Nameid2 = MyNameLink.Nameid2
WHERE MyNameLink.Nameid1 IS NULL AND MyNameLink.Nameid2 IS NULL
UNION ALL
SELECT nl2.* FROM NAMELINK nl1 INNER JOIN NAMELINK nl2 on
nl1.Nameid2 = nl2.Nameid1 and nl1.Nameid1= nl2.Nameid2
WHERE nl2.Nameid1 = ( SELECT TOP 1 nl3.Nameid1 FROM NAMELINK nl3 INNER JOIN
NAMELINK nl4 ON nl3.Nameid2 = nl4.nameid1 and nl3.Nameid1 = nl4.Nameid2
WHERE (nl3.Nameid1 + nl3.Nameid2) = (nl1.Nameid1 + nl1.Nameid2) ) and
nl2.Nameid2 = ( SELECT TOP 1 nl3.Nameid2 FROM NAMELINK nl3 INNER JOIN
NAMELINK nl4 ON nl3.Nameid2 = nl4.nameid1 and nl3.Nameid1 = nl4.Nameid2
WHERE (nl3.Nameid1 + nl3.Nameid2) = (nl1.Nameid1 + nl1.Nameid2))

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This was first published in December 2002

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