TSQL query to delete all duplicate records but one [duplicate] - tsql

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Closed 10 years ago.
Possible Duplicate:
delete duplicate records in SQL Server
I have a table in which unique records are denoted by a composite key, such as (COL_A, COL_B).
I have checked and confirmed that I have duplicate rows in my table by using the following query:
select COL_A, COL_B, COUNT(*)
from MY_TABLE
group by COL_A, COL_B
having count(*) > 1
order by count(*) desc
Now, I would like to remove all duplicate records but keep only one.
Could someone please shed some light on how to achieve this with 2 columns?
EDIT:
Assume the table only has COL_A and COL_B

Answer 1

1st solution,
It is flexible, because you can add more columns than COL_A and COL_B :
-- create table with identity filed
-- using idenity we can decide which row we can delete
create table MY_TABLE_COPY
(
id int identity,
COL_A varchar(30),
COL_B varchar(30)
/*
other columns
*/
)
go
-- copy data
insert into MY_TABLE_COPY (COL_A,COL_B/*other columns*/)
select COL_A, COL_B /*other columns*/
from MY_TABLE
group by COL_A, COL_B
having count(*) > 1
-- delete data from MY_TABLE
-- only duplicates (!)
delete MY_TABLE
from MY_TABLE_COPY c, MY_TABLE t
where c.COL_A=t.COL_A
and c.COL_B=t.COL_B
go
-- copy data without duplicates
insert into MY_TABLE (COL_A, COL_B /*other columns*/)
select t.COL_A, t.COL_B /*other columns*/
from MY_TABLE_COPY t
where t.id = (
select max(id)
from MY_TABLE_COPY c
where t.COL_A = c.COL_A
and t.COL_B = c.COL_B
)
go
2nd solution
If you have really two columns in MY_TABLE you can use:
-- create table and copy data
select distinct COL_A, COL_B
into MY_TABLE_COPY
from MY_TABLE
-- delete data from MY_TABLE
-- only duplicates (!)
delete MY_TABLE
from MY_TABLE_COPY c, MY_TABLE t
where c.COL_A=t.COL_A
and c.COL_B=t.COL_B
go
-- copy data without duplicates
insert into MY_TABLE
select t.COL_A, t.COL_B
from MY_TABLE_COPY t
go

Answer 2

Try:
-- Copy Current Table
SELECT * INTO #MY_TABLE_COPY FROM MY_TABLE
-- Delte all rows from current able
DELETE FROM MY_TABLE
-- Insert only unique values, removing your duplicates
INSERT INTO MY_TABLE
SELECT DISTINCT * FROM #MY_TABLE_COPY
-- Remove Temp Table
DROP TABLE #MY_TABLE_COPY
That should work as long as you don't break any foreign keys when deleting rows from MY_TABLE.