mysql统计查询按天，月的数据，该天，月没有数据自动补0

mysql查询最近10天的数据，没有数据自动补0; 正常的数据库中的表, 某一天如果没有数据的话, 以ctime字段为基准进行数据统计就会出现订单量为0的统计不到数据;
原网址：https://blog.csdn.net/sq_com/article/details/84997563

解决方法:

1. 想方设法的创建一张虚拟的数据表, 专门表示日期.如下:

SELECT
    tbl._date AS _date,
    IFNULL(tbr.totalnum, 0) AS totalnum
FROM
    (
        SELECT
            @s :=@s + 1 AS _index,
            DATE(
                DATE_SUB(CURRENT_DATE, INTERVAL @s DAY)
            ) AS _date
        FROM
            information_schema.tables,
            # mysql.help_topic,
            # 关联任意一张系统表; 一定的是系统表;
            (SELECT @s := - 1) temp
        WHERE
            @s < 10
        ORDER BY
            _date
    ) AS tbl
LEFT JOIN (
    SELECT   count(*) AS totalnum,  DATE(ctime) createdate
    FROM zxx_news    GROUP BY  createdate  ) AS tbr ON tbl._date = tbr.createdate
GROUP BY    tbl._date ;

2.在做报表数据统计的时候，用一个折线图用来显示并统计最近 6 个月内的数据结算情况。为了前端小伙伴方便处理数据，需要将没有数据的月份默认填充为 0

SELECT
        CONVERT (t2.year_month_str, CHAR) `timeStamp`,
    IFNULL(SUM(i.id), 0) AS total  
FROM
    (
        SELECT
            @rownum :=@rownum + 1 AS num,
            date_format(
                DATE_SUB(now(), INTERVAL @rownum MONTH),
                '%Y/%m'
            ) AS year_month_str
        FROM
            (SELECT @rownum := - 1) AS r_init,
            (
                SELECT
                    c.id
                FROM
                    zxx_news c
                LIMIT 6
            ) AS c_init
    ) t2
LEFT JOIN zxx_news AS i ON (
    CONCAT(
        DATE_FORMAT(i.ctime, '%Y'),
        '/',
        DATE_FORMAT(i.ctime, '%m')
    ) = t2.year_month_str
    AND i.id > 100
)
GROUP BY
    t2.year_month_str