使用JOIN在MySQL中创建第三列的类似查询

使用这个，我一直在尝试使用不同类型的连接来组合这两个SELECT查询。它们非常相似，各自工作也很好，但当我试图创建第三列EmpFirstName3时，查询失败了。如何成功地组合这两个表

HTG_TechProps表

HTG_技术人员设置表

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试试这个：

    SELECT 
        p.EmpNumber,
        p.EmpFirstName AS EmpFirstName1,      
        t.EmpFirstName AS EmpFirstName2
    FROM HTG_TechProps p 
    LEFT JOIN HTG_TechStaffSets s ON p.EmpNumber=s.EmpNumber
    LEFT JOIN HTG_TechProps t ON t.EmpNumber=s.StaffSetID

UNION ALL

SELECT 
    p.EmpNumber,
    p.EmpFirstName AS EmpFirstName1,      
    t.EmpFirstName AS EmpFirstName2
FROM HTG_TechProps p 
LEFT JOIN HTG_TechStaffSets s ON p.EmpNumber=s.StaffSetID
LEFT JOIN HTG_TechProps t ON t.EmpNumber=s.EmpNumber

ORDER BY p.EmpNumber

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我根据在Stack上找到的问题尝试了这个方法，但我显然是搞错了。你能冒昧给我一个样品吗？我完全迷路了。您将techProps连接到EMPNAME上的人员集，但从样本数据来看，两个表之间的EMPNAME不相同。然后你加入EMPNAME和staffsetid。我知道，它完全被抬高了。我无法控制桌子。在某些情况下，StaffSetID与EmpNumber匹配，而在某些情况下，它与EmpNumber不匹配……这足以让我发疯了，LOLOh，伙计，谈论我的复杂化……我在尝试内部连接和联合查询。非常感谢！

EmpNumber     |     StaffSetID

CCN31         |     111

CCN11         |     222

POWW          |     Null

/* Techs */
    SELECT 
        p.EmpNumber,
        p.EmpFirstName AS EmpFirstName1,      
        t.EmpFirstName AS EmpFirstName2
    FROM HTG_TechProps p 
    LEFT JOIN HTG_TechStaffSets s ON p.EmpNumber=s.EmpNumber
    LEFT JOIN HTG_TechProps t ON t.EmpNumber=s.StaffSetID
    ORDER BY p.EmpNumber


/* Staff Sets */


SELECT 
    p.EmpNumber,
    p.EmpFirstName AS EmpFirstName1,      
    t.EmpFirstName AS EmpFirstName2
FROM HTG_TechProps p 
LEFT JOIN HTG_TechStaffSets s ON p.EmpNumber=s.StaffSetID
LEFT JOIN HTG_TechProps t ON t.EmpNumber=s.EmpNumber
ORDER BY p.EmpNumber

    SELECT 
        p.EmpNumber,
        p.EmpFirstName AS EmpFirstName1,      
        t.EmpFirstName AS EmpFirstName2
    FROM HTG_TechProps p 
    LEFT JOIN HTG_TechStaffSets s ON p.EmpNumber=s.EmpNumber
    LEFT JOIN HTG_TechProps t ON t.EmpNumber=s.StaffSetID

UNION ALL

SELECT 
    p.EmpNumber,
    p.EmpFirstName AS EmpFirstName1,      
    t.EmpFirstName AS EmpFirstName2
FROM HTG_TechProps p 
LEFT JOIN HTG_TechStaffSets s ON p.EmpNumber=s.StaffSetID
LEFT JOIN HTG_TechProps t ON t.EmpNumber=s.EmpNumber

ORDER BY p.EmpNumber