SQLite 3.27

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CREATE TABLE tablename (`n` INTEGER, `x` TEXT, `y` TEXT);
✓
INSERT INTO tablename (`n`, `x`, `y`) VALUES
  ('1', 'aaa', 'foo'),
  ('2', '', 'bar'),
  ('3', '', 'baz'),
  ('4', 'bbb', 'able'),
  ('5', '', 'baker');
✓
UPDATE tablename AS t1
SET x = (
  SELECT t2.x 
  FROM tablename t2 
  WHERE t2.n < t1.n AND COALESCE(t2.x, '') <> '' 
  ORDER BY t2.n DESC LIMIT 1
)
WHERE COALESCE(t1.x, '') = '';
✓
SELECT * FROM tablename;


n
x
y


1
aaa
foo

2
aaa
bar

3
aaa
baz

4
bbb
able

5
bbb
baker