如果使用MySQL在数据库中不存在数据，如何获取计数0

Question

我正在显示在我的应用程序上注册的每月用户。为此，我使用了以下查询，效果很好。但是通过此查询，如果6月没有用户注册，则6月没有数据。我希望将6月和其他所有信息都设置为0。有人可以帮我吗？

SELECT Month(createdon), count(*) as users,COUNT(if(roleid=1,1,NULL)) as instructor, COUNT(if(roleid=2,1,NULL)) as student FROM user_profile where Year(createdon) = Year(Now()) group by MONTH(createdon);

我得到的输出为：

Month(created on) | users | instructor | student | 3 | 4 | 3 | 1 | 4 | 7 | 5 | 2 |

在这里，第3个月和第4个月分别对应于3月和4月。

但是实际输出是：

Month(created on) | users | instructor | student | 1 | 0 | 0 | 0 | 2 | 0 | 0 | 0 | 3 | 4 | 3 | 1 | 4 | 7 | 5 | 2 | 5 | 0 | 0 | 0 | 6 | 0 | 0 | 0 |

Answer 1

你可以试试这个：

SELECT MONTH(createdon) AS month, COUNT(*) AS users, COUNT(IF(roleid=1,1,NULL)) AS instructor, COUNT(IF(roleid=2,1,NULL)) AS student
FROM user_profile
WHERE (YEAR(createdon) = YEAR(NOW()))
GROUP BY MONTH(createdon)
UNION
SELECT M.month, 0, 0, 0
FROM (SELECT 1 AS month UNION SELECT 2 UNION SELECT 3 UNION SELECT 4 UNION SELECT 5 UNION SELECT 6 UNION SELECT 7 UNION SELECT 8 UNION SELECT 9 UNION SELECT 10 UNION SELECT 11 UNION SELECT 12) AS M
WHERE (M.month <= MONTH(NOW()))
  AND (NOT EXISTS (SELECT * FROM user_profile WHERE (MONTH(createdon) = M.month)));