I will take an example to explain this tip.
imagine we want to export our drupal users with their roles. if a user has 3 roles, the result of the join query will be 3 rows for him.
Using GROUP_CONCAT will give us all roles in one single row.
SELECT u.uid, u.name, u.mail,
GROUP_CONCAT(DISTINCT r.name SEPARATOR '/') as roles
from users u
inner join users_roles ur on ur.uid = u.uid
inner join role r using(rid)
GROUP BY u.uidnotice here the GROUP BY u.uid (otherwise all users will be concatenated in 1 row).