SQL Server duplicate row - sql

Question

I have a table with duplicate records. I want to mark whether the record is a duplicate or not in a another column, let's say a column name Flag. If the records is a duplicate mark it as 1 in Flag column else 0.
How to do this?
I can use a query to select duplicate records.
select
o.clientid, oc.dupeCount, o.pannodesc, o.CustNo
from
CustomerMaster1 o
inner join
(SELECT clientid, COUNT(*) AS dupeCount
FROM CustomerMaster1
WHERE ISNULL(PanNoDesc, '') <> ''
GROUP BY clientid
HAVING COUNT(*) > 1) oc ON o.clientid = oc.clientid
Simply saying, if there are two similar records, mark 1 against the second duplicated row, if three similar records mark 1 against two rows, leaving the original record as 0.

Answer 1

Just use count(*) as a window function to calculate the flag:
select o.clientid, oc.dupeCount, o.pannodesc, o.CustNo,
(case when count(*) over (partition by clientId) > 1
then 1 else 0
end) as IsDuplicate
from CustomerMaster1 o;
If you only case about certain records, then you can count them instead:
select o.clientid, oc.dupeCount, o.pannodesc, o.CustNo,
(case when sum(case when PanNoDesc <> '' or PanNoDesc is not null
then 1 else 0
end) over (partition by clientId) > 1
then 1 else 0
end) as IsDuplicate
from CustomerMaster1 o;
EDIT:
If you want to modify the data, assuming you have a flag, you can just use these statements as a CTE:
with toupdate as (
select o.clientid, oc.dupeCount, o.pannodesc, o.CustNo,
(case when sum(case when PanNoDesc <> '' or PanNoDesc is not null
then 1 else 0
end) over (partition by clientId) > 1
then 1 else 0
end) as NewIsDuplicate
from CustomerMaster1 o
)
update toupdate
set Flag = NewIsDuplicate;

Answer 2

You can write as
CREATE TABLE CustomerMaster1 (clientid INT,PanNoDesc VARCHAR(10),DupFlag bit)
INSERT INTO CustomerMaster1 VALUES(1,'A',NULL ),(1,'B',NULL )
SELECT clientid,PanNoDesc,DupFlag FROM CustomerMaster1
;WITH CTE AS(
SELECT clientid,
ROW_NUMBER()OVER (PARTITION BY clientid ORDER BY clientid ASC) AS rownum
FROM CustomerMaster1
WHERE ISNULL(PanNoDesc, '') <> ''
)
UPDATE T
SET T.DupFlag = (case WHEN rownum > 1 THEN 1 ELSE 0 END)
FROM CustomerMaster1 T
JOIN CTE ON CTE.clientid = T.clientid
SELECT clientid,PanNoDesc,DupFlag FROM CustomerMaster1
demo
Edit: Demo based on sample fields provided:
http://sqlfiddle.com/#!3/4592f/1