Expression #6 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'anribg_utf8.i.bi_filename' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
SELECT
b_id,
b_pub_name,
cit.c_name AS city,
kvr.k_name AS kvartal,
str.u_name AS street,
bi_filename
FROM
tbl_buildings
LEFT JOIN tbl_building_images AS i ON b_id = i.bi_building_id AND bi_pos = 0
LEFT JOIN tbl_kvartali AS kvr ON b_kvartal_id = kvr.k_id
LEFT JOIN tbl_ulici AS str ON b_street_id = u_id
JOIN tbl_cities AS cit ON c_id = b_city_id
GROUP BY
b_id
ORDER BY
b_pub_name ASC