Postgres 9.5

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CREATE TABLE tablename (Id INTEGER, Letter VARCHAR(1), R_Value INTEGER);

INSERT INTO tablename (Id, Letter, R_Value) VALUES
  ('1', 'A', '1500'),
  ('2', 'A', '1400'),
  ('4', 'B', '800'),
  ('9', 'B', '700'),
  ('10', 'B', '600'),
  ('11', 'A', '400'),
  ('12', 'A', '200');

INSERT 0 7


SELECT Letter, MAX(R_Value) Max_RValue
FROM (
  SELECT *, SUM(flag::int) OVER (ORDER BY R_Value DESC) grp
  FROM (
    SELECT *, Letter <> LAG(Letter, 1, '') OVER (ORDER BY R_Value DESC) flag
    FROM tablename
  ) t  
) t  
GROUP BY grp, Letter


letter
max_rvalue


A
1500

B
800

A
400



SELECT 3


SELECT Letter, MAX(R_Value) Max_RValue
FROM (
  SELECT *, SUM(flag::int) OVER (ORDER BY R_Value DESC) grp
  FROM (
    SELECT *, Letter <> LAG(Letter, 1, '') OVER (ORDER BY R_Value DESC) flag
    FROM tablename
  ) t  
) t  
GROUP BY grp, Letter


letter
max_rvalue


A
1500

B
800

A
400



SELECT 3


SELECT Letter, R_Value
FROM (
  SELECT *, LAG(Letter, 1, '') OVER (ORDER BY R_Value DESC) prev_Letter
  FROM tablename
) t
WHERE Letter <> prev_Letter


letter
r_value


A
1500

B
800

A
400



SELECT 3