UPDATE POPULATION
SET DISTRICT_NAME = NULLIF(DISTRICT_NAME, '');
SELECT 
DISTRICT_CODE AS 地区コード
, COALESCE(DISTRICT_NAME, '不明') AS 地区名
, TOTAL_AMT AS 総数
, MALE_AMT AS 男性数
, FEMALE_AMT AS 女性数
from POPULATION AS 人口;