mysql - 使用 MySQL 为交叉表提取第二个值

Question

我有以下查询...

SELECT COUNT(Department) as DeptCount, Survey.Department, Department.DepartmentName 
FROM Survey JOIN Department ON Survey.Department = Department.DeptID 
Where SurveyCompleted is not null GROUP BY DEPARTMENT ORDER BY Department;

唯一的问题是我计算的“部门”字段并不总是输入值(出于各种原因)。我可以使用另一个字段 Employee.DepartmentId，但我不确定如果我使用内联 IF 或类似结构如何创建相同的效果。

基本上我希望看​​到的输出是:

DeptCount    Department    DepartmentName
43           2             Department1
68           9             Department2
83           13            Department3

现在我得到一个额外的行作为第一个结果

DeptCount    Department    DepartmentName
93           0             <blank>
43           2             Department1
68           9             Department2
83           13            Department3

如有任何帮助，我们将不胜感激。

谢谢。

Answer 1

如果你尝试这个会发生什么？通过使连接成为左连接，我们从 Survey 中获取所有记录，并从 Department 中获取所有匹配记录。然后我们可以聚合它们。

SELECT COUNT(Department) as DeptCount, coalesce(Survey.Department, 0)
, Department.DepartmentName 
FROM Survey 
left JOIN Department ON Survey.Department = Department.DeptID
Where SurveyCompleted is not null 
GROUP BY DEPARTMENT ORDER BY Department;

顺便说一下，group by 会自动排序，因此如果您要分组，则不需要order by。