CREATE TABLE source
AS
  SELECT *
  FROM ( VALUES
    ( 6, 1000 ),
    ( 6, 2000 ),
    ( 7, 5000 ),
    ( 7, 6000 )
  ) AS t(client_id, score);

client_id | score
----+---------+---
 6         | 3000
 7         | 11000

CREATE VIEW desired_view
AS
  SELECT client_id, sum(score)
  FROM source
  GROUP BY client_id;

client_id |  sum
-----------+-------
         6 |  3000
         7 | 11000
(2 rows)