MySQL按年、月、日统计数据

SELECT
    DATE_FORMAT(created_time, '%Y-%m-%d') created_day,
    COUNT(1) orders
FROM
    p_order
GROUP BY
    created_day 
ORDER BY created_day DESC;

DATE_FORMAT：数据库自带函数

%Y-%m-%d %H：按时查询
%Y-%m-%d：按天查询
%Y-%m：按月查询