Question

I have multiple groups of duplicates in one table (3 records for one, 2 for another, etc) – multiple rows where more than 1 exists.

Below is what I came up with to delete them, but I have to run the script for however many duplicates there are:

set rowcount 1
delete from Table
where code in (
  select code from Table 
  group by code
  having (count(code) > 1)
)
set rowcount 0

This works well to a degree. I need to run this for every group of duplicates, and then it only deletes 1 (which is all I need right now).

Answer 1

If you have a key column on the table, then you can use this to uniquely identify the “distinct” rows in your table.

Just use a sub query to identify a list of ID’s for unique rows and then delete everything outside of this set. Something along the lines of…..

create table #TempTable
(
    ID int identity(1,1) not null primary key,
    SomeData varchar(100) not null
)

insert into #TempTable(SomeData) values('someData1')
insert into #TempTable(SomeData) values('someData1')
insert into #TempTable(SomeData) values('someData2')
insert into #TempTable(SomeData) values('someData2')
insert into #TempTable(SomeData) values('someData2')
insert into #TempTable(SomeData) values('someData3')
insert into #TempTable(SomeData) values('someData4')

select * from #TempTable

--Records to be deleted
SELECT ID
FROM #TempTable
WHERE ID NOT IN
(
    select MAX(ID)
    from #TempTable
    group by SomeData
)

--Delete them
DELETE
FROM #TempTable
WHERE ID NOT IN
(
    select MAX(ID)
    from #TempTable
    group by SomeData
)

--Final Result Set
select * from #TempTable

drop table #TempTable;

Alternatively you could use a CTE for example:

WITH UniqueRecords AS
(
    select MAX(ID) AS ID
    from #TempTable
    group by SomeData
)
DELETE A
FROM #TempTable A
    LEFT outer join UniqueRecords B on
        A.ID = B.ID
WHERE B.ID IS NULL