SQLite 3.27

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CREATE TABLE tablename (`students` TEXT, `grades` REAL);
✓
INSERT INTO tablename (`students`, `grades`) VALUES
  ('Nick', '34'),
  ('Nick', '42'),
  ('Nick', '86'),
  ('Nick', Null),
  ('John', '38'),
  ('John', '12'),
  ('John', '74'),
  ('John', Null),
  ('Colin', '87'),
  ('Colin', '23'),
  ('Colin', '46'),
  ('Colin', '42');
✓
UPDATE tablename AS t1
SET grades = (
  SELECT ROUND(AVG(t2.grades), 1) 
  FROM tablename AS t2 
  WHERE t2.students = t1.students
)
WHERE t1.grades IS NULL;
✓
SELECT * FROM tablename;


students
grades


Nick
34

Nick
42

Nick
86

Nick
54

John
38

John
12

John
74

John
41.3

Colin
87

Colin
23

Colin
46

Colin
42

students	grades
Nick	34
Nick	42
Nick	86
Nick	54
John	38
John	12
John	74
John	41.3
Colin	87
Colin	23
Colin	46
Colin	42