Comparing strings in sql
What you can do is something like this (see the fiddle here):
Table:
CREATE TABLE t
(
  user VARCHAR (10) NOT NULL,
  permission VARCHAR (10) NOT NULL
);

Data:
INSERT INTO t VALUES
('user1', 'a'),
('user2', 'a, b, f'),
('user3', 'b, c'),
('user4', 'e, f'),
('user5', 'f, g');

Query:
SELECT * FROM t
WHERE permission LIKE '%a%'
OR    permission LIKE '%b%'
OR    permission LIKE '%c%'
OR    permission LIKE '%d%'
OR    permission LIKE '%e%';

Result:
user    permission
user1   a
user2   a, b, f
user3   b, c
user4   e, f

user5  is excluded because his permissions are 'f,g' and these don't appear in the sequence a-e.
p.s. welcome to the forum!