How do I remove duplicates in paging - sql

Question

table1 & table2:
table1 & table2 http://aftabfarda.parsfile.com/1.png
SELECT *
FROM (SELECT DISTINCT dbo.tb1.ID, dbo.tb1.name, ROW_NUMBER() OVER (ORDER BY tb1.id DESC) AS row
FROM dbo.tb1 INNER JOIN
dbo.tb2 ON dbo.tb1.ID = dbo.tb2.id_tb1) AS a
WHERE row BETWEEN 1 AND 7
ORDER BY id DESC
Result:
Result... http://aftabfarda.parsfile.com/3.png
(id 11 Repeated 3 times)
How can I have this output:
ID name row
-- ------ ---
11 user11 1
10 user10 2
9 user9 3
8 user8 4
7 user7 5
6 user6 6
5 user5 7

Answer 1

You could apply distinct before row_number using a subquery:
select *
from (
select row_number() over (order by tbl.id desc) as row
, *
from (
select distinct t1.ID
, tb1.name
from dbo.tb1 as t1
join dbo.tb2 as t2
on t1.ID = t2.id_tb1
) as sub_dist
) as sub_with_rn
where row between 1 and 7

Answer 2

Alternatively to #Andomar's suggestion, you could use DENSE_RANK instead of ROW_NUMBER and rank the rows first (in the subquery), then apply DISTINCT (in the outer query):
SELECT DISTINCT
ID,
name,
row
FROM (
SELECT
t1.ID,
t1.name,
DENSE_RANK() OVER (ORDER BY t1.ID DESC) AS row
FROM dbo.tb1 t1
INNER JOIN dbo.tb2 t2 ON t1.ID = t2.id_tb1
) AS a
WHERE row BETWEEN 1 AND 7
ORDER BY ID DESC
Similar, but not quite the same, although both might boil down to the same query plan, I'm just not sure. Worth testing, I think.
And, of course, you could also try a semi-join instead of a proper join, in the form of either IN or EXISTS, to prevent duplicates in the first place:
SELECT
ID,
name,
row
FROM (
SELECT
ID,
name,
ROW_NUMBER() OVER (ORDER BY ID DESC) AS row
FROM dbo.tb1
WHERE ID IN (SELECT id_tb1 FROM dbo.tb2)
/* Or:
WHERE EXISTS (
SELECT *
FROM dbo.tb2
WHERE id_tb1 = dbo.tb1.ID
)
*/
) AS a
WHERE row BETWEEN 1 AND 7
ORDER BY ID DESC