SQLite 3.27

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CREATE TABLE tablename (`project_id` INTEGER, `user_id` INTEGER, `can_read_by` TEXT);
INSERT INTO tablename (`project_id`, `user_id`, `can_read_by`) VALUES
  ('1', '7', '2,3,4'),
  ('2', '6', '3'),
  ('3', '2', '5'),
  ('4', '1', '2'),
  ('5', '5', '3,1');
SELECT *
FROM tablename
WHERE user_id = '2'
   OR ',' || can_read_by || ',' LIKE '%,' || '2' || ',%';


project_id
user_id
can_read_by


1
7
2,3,4

3
2
5

4
1
2