An Error Was Encountered

Error Number: 1055

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

SELECT t.num AS num, t.titulo AS titulo, t.data AS data, i.foto_name AS foto_name, i.titulo AS foto_desc FROM site_content_txt AS t LEFT JOIN site_images AS i ON (t.num = i.gal_id AND i.tipo = 'image') WHERE t.tipo = 'calendar' AND t.active = 'yes' AND destaque = 'no' GROUP BY t.num ORDER BY t.data DESC LIMIT 20