按月列出分组数据，包括没有结果的月份

Question

Hello community 以下查询将每个活动状态的出现次数相加，然后按月分组

select
    date_format(date_created, '%b') month,
    month(date_created) pivot,
    sum(case when a.state = 'created' then 1 else 0 end) created,
    sum(case when a.state = 'notified' then 1 else 0 end) notified,
    sum(case when a.state = 'confirmed' then 1 else 0 end) confirmed,
    sum(case when a.state = 'approved' then 1 else 0 end) approved,
    sum(case when a.state = 'authorized' then 1 else 0 end) authorized,
    sum(case when a.state = 'attended' then 1 else 0 end) attended,
    sum(case when a.state = 'canceled' then 1 else 0 end) canceled,
    count(a.id) as total
from activities a
group by 1 order by pivot desc;

所以我得到以下结果

我想在这个结果中添加没有数据的月份，并用零自动完成状态总和

我分享这个sqlfiddle

我发现这个answer 用于类似的场景，但我不明白如何将它应用到我向您展示的案例中

非常感谢您的帮助

Answer 1

您可以使用日期表（子查询）和LEFT JOIN：

select
    s.name month,
    s.m pivot,
    sum(case when a.state = 'created' then 1 else 0 end) created,
    sum(case when a.state = 'notified' then 1 else 0 end) notified,
    sum(case when a.state = 'confirmed' then 1 else 0 end) confirmed,
    sum(case when a.state = 'approved' then 1 else 0 end) approved,
    sum(case when a.state = 'authorized' then 1 else 0 end) authorized,
    sum(case when a.state = 'attended' then 1 else 0 end) attended,
    sum(case when a.state = 'canceled' then 1 else 0 end) canceled,
    count(a.id) as total
from (SELECT  1 m, 'Jan' AS name 
      UNION SELECT 2, 'Feb' 
      UNION SELECT 3, 'Mar'
      UNION ...) s
LEFT JOIN activities a
  ON s.m = month(date_created)
  --AND s.y = year(date_created)   -- if needed 
group by 1
order by pivot desc;

DBFiddle Demo