My tables are these :
Employee Table:
+-----------+----------+------------+ | id | name | department | +-----------+----------+-----------
You should avoid storing data as comma-separated values, and follow normalization.
However in this case you can do something as
select e.id , e.name , group_concat(d.name) from employee e left join department d on find_in_set(d.id,e.department) group by e.id ;