Sql 使用别名表选择同一列两次

下面是我正在使用的表格示例。这些仅表示与我的查询相关的列

_要求

    RequirementID             fkOwningWsID
    -------------------------------------------    
    REQ-RPT-01                1
    REQ-RPT-02                2

_工作流

    pk             WsNm
    -------------------------------------------    
    1               Workstream1
    2               Workstream2

MNU工作流程领导

    fkWsID          fkEeID
    -------------------------------------------    
    1               1
    1               2
    2               1
    2               2

下表是联合的结果。员工可以来自不同的公司，下面的工会列出了所有员工ID、来自公司1的员工ID（否则为0）和来自公司2的员工ID（否则为0）

qrytrackerleelist

    EeID             Company1_ID           Company2_ID
    -------------------------------------------    
    1                 1                     0
    2                 0                     2

我正在尝试查看以下结果

    RequirementID        WsNm          Company1_Lead      Company2_Lead
    --------------------------------------------------------------------   
    REQ-RPT-01           Workstream1    1                  2
    REQ-RPT-02           Workstream2    1                  2

我发布了以下SQL语句

 SELECT DISTINCT Req.RequirementID,  Ws.Wsnm, company1_id.ee_id, company2_id.ee_id
FROM (((([_Requirements] AS Req 
INNER JOIN [_Workstream] AS Ws ON Req.fkOwningWsID = Ws.pkWsID)
INNER JOIN [mnWorkstream_Leads] AS wsLeads ON Ws.pkWsID = wsLeads.fkWsID)
LEFT OUTER JOIN qryTrackerAllEeList AS company1 ON wsLeads.fkEeID = company1.Company1_ID)
LEFT OUTER JOIN qryTrackerAllEeList AS company2 ON wsLeads.fkEeID = company2.Company2_ID)

然而，问题是，我检索以下结果

    RequirementID        WsNm          Company1_Lead      Company2_Lead
    --------------------------------------------------------------------   
    REQ-RPT-01           Workstream1                       2
    REQ-RPT-01           Workstream1    1                  
    REQ-RPT-02           Workstream2                       2    
    REQ-RPT-02           Workstream2    1

关于如何消除这些重复行和空值的任何建议？

使用

MAX（）

和

groupby

仅选择非空值并将其分组为一行：

SELECT DISTINCT Req.RequirementID,  Ws.Wsnm, 
MAX(company1_id.ee_id) as Company1_Lead, MAX(company2_id.ee_id) as Company2_Lead,
FROM (((([_Requirements] AS Req 
INNER JOIN [_Workstream] AS Ws ON Req.fkOwningWsID = Ws.pkWsID)
INNER JOIN [mnWorkstream_Leads] AS wsLeads ON Ws.pkWsID = wsLeads.fkWsID)
LEFT OUTER JOIN qryTrackerAllEeList AS company1 ON wsLeads.fkEeID = company1.Company1_ID)
LEFT OUTER JOIN qryTrackerAllEeList AS company2 ON wsLeads.fkEeID = company2.Company2_ID)
GROUP BY req.RequirementID, Ws.Wsnm