An Error Was Encountered

Error Number: 1055

Expression #4 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'sel.foto_name' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

SELECT *, GROUP_CONCAT( DISTINCT tag ) AS tags FROM ( SELECT t.num, t.titulo, t.texto, i.foto_name,, tt.tag FROM site_content_txt AS t LEFT JOIN site_images AS i ON t.num = i.gal_id LEFT JOIN site_content_to_tags AS ct ON t.num = ct.post_id LEFT JOIN site_tags AS tt ON ct.tag_id = tt.tag_id WHERE t.tipo = 'blog' AND = 'yes' ORDER BY i.position ASC ) AS sel GROUP BY num ORDER BY data DESC, num DESC LIMIT 1000000