如何在SQL中应用以下情况下的条件聚合？

Question

我有2表：

1. tbDepatrment 2. tbEmployee

结果所需要的：

Department  Employees(by department) Male(By department)  Female(by department) 
Employee   11       6       5 
Management  10       5       5 
Owner   1        1       0 

我用这个：

SELECT [tbDepartment].[DepartmentName] AS [Department name], 
     Count(*) AS [Number of employees by department type], 
     (Select Count([EmployeeGender]) FROM [tbEmployee] WHERE [EmployeeGender] = 'Male' AND [DepartmentName] = [tbDepartment].[DepartmentName]) AS [Number of male employees by department type], 
     (Select Count([EmployeeGender]) FROM [tbEmployee] WHERE [EmployeeGender] = 'Female' AND [DepartmentName] = [tbDepartment].[DepartmentName]) AS [Number of female employees by department type] 
FROM [tbEmployee] JOIN [tbDepartment] 
ON [tbEmployee].[DepartmentId] = [tbDepartment].[DepartmentId] 
GROUP BY [tbDepartment].[DepartmentName]; 

导致为：

Department  Employees(by department) Male(By department)  Female(by department) 
Employee   11       12       10 
Management  10       12       10 
Owner   1        12       10 

如何根据第1列或按列分组显示第3列和第4列？

Answer 1

您不需要子查询来计算男性和女性的数量。相反，使用CASE表达式与条件聚合。

SELECT [tbDepartment].[DepartmentName] AS [Department name], 
    COUNT(*) AS [Number of employees by department type], 
    SUM(CASE WHEN [EmployeeGender] = 'Male' THEN 1 ELSE 0 END) AS [Number of male employees by department type], 
    SUM(CASE WHEN [EmployeeGender] = 'Female' THEN 1 ELSE 0 END) AS [Number of female employees by department type] 
FROM [tbEmployee] JOIN [tbDepartment] 
    ON [tbEmployee].[DepartmentId] = [tbDepartment].[DepartmentId] 
GROUP BY [tbDepartment].[DepartmentName]; 

更新：

虽然上面我在回答有条件的聚集是去这里，其实我不知道为什么你的子查询被赋予了错误的输出方式。你得到的输出是所有部门中男性和女性的总数，针对每个部门。你所得到的输出将与形式的子查询是一致的：

SELECT COUNT([EmployeeGender]) 
FROM [tbEmployee] 
WHERE [EmployeeGender] = 'Male' 

即没有部门检查。也许你应该检查你的原始查询，看看它是否与你在这里发布的内容相符。