mysql 为列中每个特定值填充缺失的日期

Question

我正在尝试计算 l_events 表的 subchannel 列中 2013-10-01 和2014-03-31 并按特定子 channel 、年和月进行分组。

现在我的结果如下:

year    month   subchannel      count(*)
2013      10    creativemornings    1
2014       3    creativemornings    2
2013      11    founderinstitute    1

我希望我的结果包含每个特定子 channel 的行，其中该月没有发生任何事件。所以类似:

year    month   subchannel      count(*)
2013      10    creativemornings    1
2013      11    creativemornings    0
2013      12    creativemornings    0
2014       1    creativemornings    0
2014       2    creativemornings    0
2014       3    creativemornings    2
2013      10    founderinstitute    0
2013      11    founderinstitute    1
2013      12    founderinstitute    0
2014       1    founderinstitute    0
2014       2    founderinstitute    0
2014       3    founderinstitute    0

我尝试将查询加入日历表(其中包括所有相关日期的日期时间列)，但它不起作用。有什么想法吗？

    SELECT  
        year(l.created) as year,
        month(l.created) as month,
        l.subchannel,
        count(*)

    FROM db.l_events l

    right JOIN db.calendar c 
        ON (date(l.created) = c.datefield)

    WHERE  
         l.created between '2013-10-01' and '2014-03-31' 

    group by
        l.subchannel,
        year(l.created), 
        month(l.created)
        ;

Answer 1

这样就可以了。它会创建您的子 channel 列表和日历表的笛卡尔积，并将 l_events 表左连接到该结果集。

SELECT  
    year(c.datefield) as year,
    month(c.datefield) as month,
    s.subchannel,
    count(l.created)
FROM db.calendar c
    CROSS JOIN (select distinct subchannel from l_events) s
    LEFT JOIN db.l_events l
        ON (date(l.created) = c.datefield)
        AND l.subchannel = s.subchannel
Where c.datefield between '2013-10-01' and '2014-03-31' 
group by
    s.subchannel,
    year(c.created), 
    month(.created)
order by
    s.subchannel,
    year(c.created), 
    month(.created)
    ;