SQLite 3.39

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CREATE TABLE tablename (`name` TEXT, `nationality` TEXT);
✓
INSERT INTO tablename (`name`, `nationality`) VALUES
  ('AAA', 'french'),
  ('BBB', 'english'),
  ('CCC', 'spanish'),
  ('DDD', 'dutch'),
  ('BBB', null),
  ('AAA', null);
✓
UPDATE tablename AS t1
SET nationality = (SELECT MAX(t2.nationality) FROM tablename t2 WHERE t2.name = t1.name)
WHERE t1.nationality IS NULL;
✓
SELECT * FROM tablename;


name
nationality


AAA
french

BBB
english

CCC
spanish

DDD
dutch

BBB
english

AAA
french

`name`	`nationality`
`AAA`	`french`
`BBB`	`english`
`CCC`	`spanish`
`DDD`	`dutch`
`BBB`	`english`
`AAA`	`french`