Question

我有描述和创建日期的合格风险，谁附加到风险子类别，最后一个风险属于风险类别，每个风险都有一个名称，如风险1＆＃39; ，我的目标是按过去3个月的名称和风险类别计算风险数量。

我有这个sql请求：

SELECT MONTH(risk.creation_date) as month, count(*) as number, risk_category.name as risk_name FROM risk As risk
 JOIN risk_subcategory on risk_subcategory.id = risk.risk_subcategory_id
  JOIN risk_category on risk_category.id = risk_subcategory.risk_category_id
where risk.creation_date >= (NOW()-INTERVAL 3 MONTH) GROUP BY MONTH(risk.creation_date), risk_category.name;

返回此结果集：

      month number  risk_name
---------------------------
      12      1      Risk 1
      12      3      Risk 2
       1      1      Risk 3
       1      9      Risk 2
       2      1      Risk 3
       2      1      Risk 1
       2     10      Risk 2

我想要这个结果（包括0）：

  month number  risk_name
---------------------------
    12   1       Risk 1
    12   3       Risk 2
    12   0       Risk 3
    1    0       Risk 1
    1    1       Risk 3
    1    9       Risk 2
    2    1       Risk 3
    2    1       Risk 1
    2    10      Risk 2

我该怎么办？感谢

Answer 1

您正在从执行count的同一个表中选择月份，因此如果月份x的计数应该为0，则意味着没有包含month = x的行。你无法从那张桌子上获得那个月，因为它根本就不存在。

如果您希望获得所有月份，则应使用另一个表格，其中包含所有可用月份，并在该表格与left join之间执行risk，例如

select  t1.month, coalesce(t2.count(*), 0)), t2.risk_name
from    months t1
left join
        risk t2
on      t1.month = MONTH(risk.creation_date)
group by t1.month, t2.risk_name

Answer 2

根据投票的答案，我试过这个但是没有用：

   SELECT MONTH(risk.creation_date) as month, count(*) as number, risk_category.name as risk_name FROM risk As risk
  JOIN risk_subcategory on risk_subcategory.id = risk.risk_subcategory_id
   JOIN risk_category on risk_category.id = risk_subcategory.risk_category_id
 LEFT JOIN (
    SELECT mois
    FROM
      (
        SELECT 1 AS mois
        UNION SELECT 2 AS mois
        UNION SELECT 3 AS mois
        UNION SELECT 4 AS mois
        UNION SELECT 5 AS mois
        UNION SELECT 6 AS mois
        UNION SELECT 7 AS mois
        UNION SELECT 8 AS mois
        UNION SELECT 9 AS mois
        UNION SELECT 10 AS mois
        UNION SELECT 11 AS mois
        UNION SELECT 12 AS mois
      ) as year
    ) as months on months.mois = MONTH(risk.creation_date)
where risk.creation_date >= (NOW()-INTERVAL 3 MONTH) GROUP BY MONTH(risk.creation_date), risk_category.name;

如何获得计数（*）返回0

2 个答案: