Sql 在单个select语句中对同一列进行多次有条件求和？

我有一个单独的表，其中显示了每个月在给定位置针对各种部署的员工部署：

ID | Location_ID | Date | NumEmployees | DeploymentType_ID

例如，一些记录可能是：

 1 | L1 | 12/2010 | 7 | 1 (=Permanent)
 2 | L1 | 12/2010 | 2 | 2 (=Temp)
 3 | L1 | 12/2010 | 1 | 3 (=Support)
 4 | L1 | 01/2011 | 4 | 1
 5 | L1 | 01/2011 | 2 | 2
 6 | L1 | 01/2011 | 1 | 3
 7 | L2 | 12/2010 | 6 | 1
 8 | L2 | 01/2011 | 6 | 1
 9 | L2 | 12/2010 | 3 | 2

我需要做的是按日期对各种类型的人进行汇总，结果如下所示：

Date    | Total Perm | Total Temp | Total Supp
12/2010 |     13     |     5      |      1
01/2011 |     10     |     2      |      1

目前，我为每种部署类型创建了一个单独的查询，如下所示：

SELECT Date, SUM(NumEmployees) AS "Total Permanent"
FROM tblDeployment
WHERE DeploymentType_ID=1
GROUP BY Date;

我们将该查询称为qSumPermDeployments。然后，我将使用几个连接来组合查询：

SELECT qSumPermDeployments.Date, qSumPermDeployments.["Total Permanent"] AS "Permanent"
    qSumTempDeployments.["Total Temp"] AS "Temp"
    qSumSupportDeployments.["Total Support"] AS Support
FROM (qSumPermDeployments LEFT JOIN qSumTempDeployments 
    ON qSumPermDeployments.Date = qSumTempDeployments.Date) 
LEFT JOIN qSumSupportDeployments 
    ON qSumPermDeployments.Date = qSumSupportDeployments.Date;

请注意，我目前正在构建最终查询，假设一个位置只有临时员工或支持员工，如果他们也有永久员工。因此，我可以使用永久雇员结果作为基表来创建联接。考虑到我目前拥有的所有数据，这个假设成立，但理想情况下，我想离开这个假设

最后，我的问题。有没有一种方法可以简化为一个查询，或者最好将其分为多个查询？如果没有其他原因，那么就可以提高可读性。

尝试以下方法：

SELECT Date, 
    SUM(case when DeploymentType_ID = 1 then NumEmployees else null end) AS "Total Permanent", 
    SUM(case when DeploymentType_ID = 2 then NumEmployees else null end) AS "Total Temp", 
    SUM(case when DeploymentType_ID = 3 then NumEmployees else null end) AS "Total Supp"
FROM tblDeployment
GROUP BY Date

SELECT
    Date,
    SUM(CASE WHEN DeploymentType_ID=1 THEN NumEmployees ELSE 0 END) AS "Total Permanent",
    SUM(CASE WHEN DeploymentType_ID=2 THEN NumEmployees ELSE 0 END) AS "Total Temporary",
    SUM(CASE WHEN DeploymentType_ID=3 THEN NumEmployees ELSE 0 END) AS "Total Support"
FROM tblDeployment
GROUP BY Date;

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+1：这次只落后你11秒。。。我越来越近了。：）哇！我真的走了很长的路。谢谢你的帮助。从2017年开始谢谢你！比我想象的容易多了。