SQL - Update column for each duplicate in other column - sql

Question

I'm looking for an SQL query to update a column to the same Id for each duplicate in another column and then delete each overwritten row from another table.
For example
I have:
IndustryId ProductId ExternalId
144 3332 13
147 3423 13
148 3532 13
2637 63199 32
121 2789 32
I want:
IndustryId ProductId ExternalId
144 3332 13
147 3332 13
148 3332 13
2637 63199 32
121 63199 32
I also need to record the overwritten ProductId values so that I can delete those rows in another table (3423, 3532, and 2789 should be deleted in the other table). Could be in more than one query if neccesary, that doesnt matter.
What is the best way to achieve this?

Answer 1

That isn't too tough, especially if you break out the steps.
Check this out:
if object_id('tempdb..#myData') is not null
drop table #myData;
create table #myData ( industryid int, productId int, externalId int);
insert into #myData (industryId, productId, externalId)
values (144,3332,13);
insert into #myData (industryId, productId, externalId)
values (147,3423,13);
insert into #myData (industryId, productId, externalId)
values (148,3532,13);
insert into #myData (industryId, productId, externalId)
values (2637,63199,32);
insert into #myData (industryId, productId, externalId)
values (121,2789,32);
--select * from #myData;
-------------------------------------------------
if object_id('tempdb..#IdsToKeep') is not null
drop table #IdsToKeep;
if object_id('tempdb..#badRows') is not null
drop table #badRows;
create table #IdsToKeep (externalId int, productId int);
create table #badRows ( industryId int, productId int, externalId int);
insert into #IdsToKeep
select
externalId, min(productId)
from
#myData
group by
externalId;
--- Capture rows that will be changed ---
insert into #badRows
select
md.industryId, md.productId, md.externalId
from
#myData md
left join #IdsToKeep itk on
md.externalId = itk.externalId
and
md.productId = itk.productId
where
itk.productId IS NULL
;
--- Make the update to the main table ---
update
#myData
set
productId = itk.productId
from
#myData
inner join #IdsToKeep itk on #myData.externalId = itk.externalId
;
----------
select * from #mydata;
select * from #badRows;

Answer 2

declare #table table (IndustryId int, ProductId int, ExternalId int) insert into #table values (144,3332,13), (147,3423,13), (148,3532,13), (2637,63199,32), (121,2789,32)
select * from #table
;with cte (productid, externalid) as (select max(productid), ExternalId from #table group by ExternalId) select t.IndustryId, c.productid, c.externalid from cte c right outer join #table t on c.externalid = t.ExternalId