如果group by不返回任何值，则显示零

尝试：

SELECt DATE_SUB(CURDATE(), INTERVAL i DAY) date_created_at,        count(id) FROM (SELECT 1 i UNIOn ALL       SELECT 2 UNIOn ALL       SELECT 3 UNIOn ALL       SELECT 4 UNIOn ALL       SELECT 5 UNIOn ALL       SELECT 6 UNIOn ALL       SELECT 7) iLEFT JOIN user_accts ON  date(created_at) = DATE_SUB(CURDATE(), INTERVAL i DAY)        GROUP BY DATE_SUB(CURDATE(), INTERVAL i DAY)