MySQL 8.0

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CREATE TABLE `function`(
   id       int NOT NULL PRIMARY KEY
  ,`function` VARCHAR(2)
  ,users    VARCHAR(20)
);

INSERT INTO `function`(id,`function`,users) VALUES ('1','f1','1,2');
INSERT INTO `function`(id,`function`,users) VALUES ('2','f2','2,3');
INSERT INTO `function`(id,`function`,users) VALUES ('3','f1','4,5');
INSERT INTO `function`(id,`function`,users) VALUES ('4','f2',NULL);
CREATE TABLE users(
   id  int NOT NULL PRIMARY KEY
  ,name VARCHAR(6)
);
INSERT INTO users(id,name) VALUES ('1','Mark');
INSERT INTO users(id,name) VALUES ('2','Louis');
INSERT INTO users(id,name) VALUES ('3','John');
INSERT INTO users(id,name) VALUES ('4','Denver');
INSERT INTO users(id,name) VALUES ('5','Nat');
select f.function, group_concat(u.name order by find_in_set(u.id, f.users)) user_names
from `function` f
left join users u on find_in_set(u.id, f.users)
group by f.id


function
user_names


f1
Mark,Louis

f2
Louis,John

f1
Denver,Nat

f2
null