mysql按照天/日分组后统计数据，没有的补0,——统计专用

步骤

1.创建日历表和数字计算表
CREATE TABLE num (i int);– 创建一个表用来储存0-9的数字
INSERT INTO num (i) VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9);– 生成0-9的数字，方便以后计算时间

CREATE TABLE if not exists calendar(datelist date);

– 生成一个存储日期的表，datalist是字段名

– 这里是生成并插入日期数据

INSERT INTO calendar(datelist) SELECT
    adddate(
        (   -- 这里的起始日期，你可以换成当前日期
            DATE_FORMAT("2016-1-1", '%Y-%m-%d') 
        ),
        numlist.id
    ) AS `date`
FROM
    (
        SELECT
            n1.i + n10.i * 10 + n100.i * 100 + n1000.i * 1000+ n10000.i * 10000 AS id
        FROM
            num n1
        CROSS JOIN num AS n10
        CROSS JOIN num AS n100
        CROSS JOIN num AS n1000
        CROSS JOIN num AS n10000
    ) AS numlist;

2.查询语句

SELECT
	dday,
	count(*) AS sum
FROM
    (
        SELECT
            datelist as dday
        FROM