T-SQL 声明子查询返回多个值但数据是唯一的

Question

第一个问题。我使用的是SQL Server 2012，在其他地方找不到类似的问题，而且我还在学习SQL。 我正在尝试让子查询返回一个值。当您比较 JobPaperID AND 里程碑时，我的表 (JobMilestones) 是唯一的。任何独立查询将仅返回一个值或不返回任何值。它给出了错误 -

子查询返回超过 1 个值。当子查询跟在 =、!=、<, <= , >、>= 后面或子查询用作表达式时，这是不允许的

当我不添加子查询时，查询的其余部分会给出正确的结果，但我也确实需要子查询。在 JobMilestones 表中，我要么有一个特定作业的条目，该条目具有唯一的 JobPaperID 和唯一的里程碑（一起），要么没有条目（我怀疑后者是问题所在）。我希望子查询返回 CompletedDate 或在记录不存在时返回单个 NULL 值。我希望它返回一个 NULL（没有记录）或 CompletedDate 的值。我哪里错了？

这是我正在使用的代码：

SELECT 
    mjd.ID as JobID,
    mjd.JobNo as JobNo, 
    mjd.CreateDateTime as JobCreatedDateTime, 
    mjd.InvoiceCustomerCode as CustomerCode, 
    mjd.MilestoneGroup as MilestoneGroup, 
    mg.Name as MilestoneGroupDescr, 
    ml.MilestoneID as MilestoneID, 
    case m.CustomType
        when 0 then m.Type
        else m.CustomType
    end AS MilestoneReal, 
    m.Name as MilestoneName,
    jp.ID as JobPaperID, 
    jp.Name as JobPaperName,
    (select jms.CompletedDate
        from dbo.JobMilestones AS jms
        where ((jms.JobPaperID = jp.ID) 
            AND (jms.Milestone = MilestoneReal))
    ) AS CompletedDate
FROM dbo.MainJobDetails AS mjd
INNER JOIN dbo.MilestoneGroup AS mg
    ON mjd.MilestoneGroup = mg.Code
INNER JOIN dbo.JobPaper AS jp 
    ON mjd.ID = jp.JobID
INNER JOIN dbo.MilestoneLink AS ml 
    ON mg.ID = ml.MilestoneGroupID
INNER JOIN dbo.Milestones AS m 
    ON ml.MilestoneID = m.ID
WHERE mjd.ID = 87101 

如果我将其更改为 SELECT TOP 1，我得到的所有内容都是 NULL，并且没有有效条目

Answer 1

感谢@Martin Smith，解决方案看起来很简单：

SELECT 
    mjd.ID as JobID,
    mjd.JobNo as JobNo, 
    mjd.CreateDateTime as JobCreatedDateTime, 
    mjd.InvoiceCustomerCode as CustomerCode, 
    mjd.MilestoneGroup as MilestoneGroup, 
    mg.Name as MilestoneGroupDescr, 
    ml.MilestoneID as MilestoneID, 
    case m.CustomType
        when 0 then m.Type
        else m.CustomType
    end AS MilestoneReal, 
    m.Name as MilestoneName,
    jp.ID as JobPaperID, 
    jp.Name as JobPaperName,
    (select jms.CompletedDate
        from dbo.JobMilestones AS jms
        where ((jms.JobPaperID = jp.ID) 
            AND (jms.Milestone = 
                    case m.CustomType
                        when 0 then m.Type
                        else m.CustomType
                    end))
    ) AS CompletedDate
FROM dbo.MainJobDetails AS mjd
INNER JOIN dbo.MilestoneGroup AS mg
    ON mjd.MilestoneGroup = mg.Code
INNER JOIN dbo.JobPaper AS jp 
    ON mjd.ID = jp.JobID
INNER JOIN dbo.MilestoneLink AS ml 
    ON mg.ID = ml.MilestoneGroupID
INNER JOIN dbo.Milestones AS m 
    ON ml.MilestoneID = m.ID
WHERE mjd.ID = 87101

唉！我无法在子查询中使用别名。