SQLite 3.39

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CREATE TABLE tablename (`x` INTEGER, `y` INTEGER);
✓
INSERT INTO tablename (`x`, `y`) VALUES
  ('255', '0'),
  ('255', '0'),
  ('5', '10'),
  ('5', '0'),
  ('5', '1'),
  ('5', '0'),
  ('255', '0'),
  ('255', '0'),
  ('2', '5'),
  ('2', '2'),
  ('2', '5'),
  ('255', '0');
✓
SELECT DISTINCT x, 
       FIRST_VALUE(y) OVER (PARTITION BY x ORDER BY COUNT(*) DESC) y
FROM tablename
GROUP BY x, y;


x
y


2
5

5
0

255
0