Как правильно обьденить строки в SQL?
В group by перечисляете все поля кроме domain_id.select 
users.name AS users_name, 
users.uid AS uid, 
users.created AS users_created,
GROUP_CONCAT(DISTINCT domain_editor.domain_id ORDER BY domain_editor.domain_id ASC SEPARATOR ',') AS domain_editor_domain_id_concated, 
'user' AS field_data_field_user_statut_user_entity_type
...
group by users_name, uid, users_created, field_data_field_user_statut_user_entity_type
GROUP BY uid и GROUP_CONCAT(domain_editor.domain_id) AS domain_editor_domain_id