如果子查询为空，则出现MySQL问题

我有5张桌子（我只谈请求中没有的）：

• 员工（登录名，代理ID）
• 频率测试（id、客户邮件、员工登录）
• 任务测试（id、客户邮件、员工登录）
• 情景测试（id、客户邮件、员工登录）
• 客户（电子邮件、年龄、性别）

这是我的要求：

SELECT Client.email, Client.age, Client.gender
FROM Client,
  (SELECT FreqTest.clientEmail
  FROM FreqTest,
    (SELECT Employee.login
    FROM Employee
    WHERE Employee.agencyID = 'anID') EmployeeByAgency
  WHERE FreqTest.employeeLogin = EmployeeByAgency.login) Freq,

  (SELECT QuestTest.clientEmail
  FROM QuestTest,
    (SELECT Employee.login
    FROM Employee
    WHERE Employee.agencyID = 'anID') EmployeeByAgency
  WHERE QuestTest.employeeLogin = EmployeeByAgency.login) Quest,

  (SELECT SitTest.clientEmail
  FROM SitTest,
    (SELECT Employee.login
    FROM Employee
    WHERE Employee.agencyID = 'anID') EmployeeByAgency
  WHERE SitTest.employeeLogin = EmployeeByAgency.login) Sit

WHERE Client.email = Freq.emailClient
OR    Client.email = Quest.emailClient
OR    Client.email = Sit.emailClient
GROUP BY Client.email;

我想要在代理机构进行测试的客户的电子邮件。 它几乎在任何情况下都有效，除非没有客户在同一机构进行了3次测试

我不得不试着用英语翻译，如果我的英语不完美，我很抱歉

因此，我要明确（与同一机构）：

• 如果客户A有频率测试和任务测试，客户B有3，客户C没有的，我会收到A和B的电子邮件
• 如果客户A进行了3次测试，客户B也进行了3次测试，客户C没有进行测试，我仍然会收到A和B的电子邮件
• 如果没有客户进行了3次不同的测试，无论我得到什么都没有

我想可能是因为如果任何子查询（Freq、Quest或Sit）都没有返回结果，那么它将是空的

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如果我不清楚或者翻译这些内容时出错，我想再次道歉。

使用左连接避免空引用

SELECT Client.email, Client.age, Client.gender
FROM Client
LEFT JOIN  (SELECT FreqTest.clientEmail
  FROM FreqTest,
    (SELECT Employee.login
    FROM Employee
    WHERE Employee.agencyID = 'anID') EmployeeByAgency
  WHERE FreqTest.employeeLogin = EmployeeByAgency.login) Freq
ON Client.email = Freq.emailClient

LEFT JOIN  (SELECT QuestTest.clientEmail
  FROM QuestTest,
    (SELECT Employee.login
    FROM Employee
    WHERE Employee.agencyID = 'anID') EmployeeByAgency
  WHERE QuestTest.employeeLogin = EmployeeByAgency.login) Quest
ON Client.email = Quest.emailClient

LEFT JOIN  (SELECT SitTest.clientEmail
  FROM SitTest,
    (SELECT Employee.login
    FROM Employee
    WHERE Employee.agencyID = 'anID') EmployeeByAgency
  WHERE SitTest.employeeLogin = EmployeeByAgency.login) Sit
ON  Client.email = Sit.emailClient
WHERE Freq.emailClient is NOT NULL
OR    Quest.emailClient IS NOT NULL
OR    Sit.emailClient IS NOT NULL
GROUP BY Client.email;

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工作如期进行，谢谢。我为笨拙地处理空值感到难过。