select * from tPerson order by ds_alias

results in

id_person | ds_name | ds_alias
    15    |  mark   |    AA
    20    |  john   |    AA
    5     |  mike   |    BB
    8     | thomas  |    JK
    2     |  mike   |    MM

-- Prepare sample data
DECLARE	@Sample TABLE (id_person INT, ds_name VARCHAR(10), ds_alias VARCHAR(10))

INSERT	@Sample
SELECT	15, 'mark', 'AA' union all
SELECT	20, 'john', 'AA' union all
SELECT	5, 'mike', 'BB' union all
SELECT	8, 'thomas', 'JK' union all
SELECT	2, 'mike', 'MM'

-- Stage some data
DECLARE	@Name VARCHAR(20)

SELECT	@Name = ds_name
FROM	@Sample
WHERE	ds_alias = 'mm'

-- Show the expected output
SELECT		COUNT(*) + 1 AS Position
FROM		@Sample
WHERE		ds_alias <= 'mm'
		AND ds_name <> @Name

SELECT	 *
FROM	 clients
WHERE	 ds_name <= 'a quatro'
	 and ds_alias <> ''
ORDER BY ds_alias
---------------------------------
cd_client  |  ds_name				 	 | ds_alias
26	   |  BANCO SANTOS NEVES		 	 | BCO STO NEVES
1857	   |  GIOVANNA GONÇALVES DIAS	 	 	 | 100% EVENTOS
1973	   |  VITÓRIA ENTRETENIMENTO LTDA	 	 | 100% VIDEO
240	   |  3 PONTOS PROPAGANDA LTDA		 	 | 3 PONTOS PROPAGANDA
2	   |  A EDICAO LIVRARIA E DIST. LTDA	 	 | A EDICAO LIVRARIA
538	   |  S/A A GAZETA			 	 | A GAZETA (JORNAL)
3	   |  A GAZETA DO ESPIRITO SANTO RADIO E TV LTDA | A GAZETA (RADIO / TV)
340	   |  A4 PUBLICIDADE E MARKETING LTDA		 | A QUATRO
2463	   |  SDC/QUATRO COMUNICAÇÃO E MARKETING LTDA	 | A QUATRO

select	cd_client (this is the id), ds_name, ds_alias,
	(select (count(c2.cd_client) +
		(select	count(c3.cd_client)
		from	clients c3
		where	c3.ds_alias <> ''
			and c3.ds_alias = c1.ds_alias
			and c3.cd_client <= c1.cd_client))
	from clients c2
	where c2.ds_alias <> ''
	and c2.ds_alias < c1.ds_alias) as Pos
from clients c1
order by c1.ds_name

SELECT		s1.cd_client,
		s1.ds_name,
		s1.ds_alias,
		(SELECT COUNT(*) FROM @Sample AS s2 WHERE s2.ds_alias < s1.ds_alias) + 1 +
		(SELECT COUNT(*) FROM @Sample AS s3 WHERE s3.ds_alias = s1.ds_alias AND s3.cd_client < s1.cd_client) AS Pos
FROM		@Sample AS s1
ORDER BY	4