MySQL 一个查询中的多个联接？

您可以像这样简单地添加另一个联接：

SELECT dashboard_data.headline, dashboard_data.message, dashboard_messages.image_id, images.filename
FROM dashboard_data 
    INNER JOIN dashboard_messages 
        ON dashboard_message_id = dashboard_messages.id
    INNER JOIN images
        ON dashboard_messages.image_id = images.image_id 

但是请注意，因为它是 INNER JOIN，如果您有没有图像的消息，则将跳过整行。如果这是可能的，您可能需要执行 LEFT OUTER JOIN ，它将返回所有仪表板消息和仅当存在时才会返回 image_filename （否则您将获得空值）

SELECT dashboard_data.headline, dashboard_data.message, dashboard_messages.image_id, images.filename
FROM dashboard_data 
    INNER JOIN dashboard_messages 
        ON dashboard_message_id = dashboard_messages.id
    LEFT OUTER JOIN images
        ON dashboard_messages.image_id = images.image_id 

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只需添加另一个联接：

SELECT dashboard_data.headline,
       dashboard_data.message,
       dashboard_messages.image_id,
       images.filename 
FROM dashboard_data 
    INNER JOIN dashboard_messages
            ON dashboard_message_id = dashboard_messages.id 
    INNER JOIN images
            ON dashboard_messages.image_id = images.image_id

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我分享了在单个 SQL 查询中使用两个 LEFT JOINS 的经验。

我有 3 张桌子：

表 1) 患者包含列 PatientID、PatientName

表 2) Appointment 包含 AppointmentID、AppointmentDateTime、PatientID、DoctorID 列

表 3) Doctor 包含 DoctorID、DoctorName 列

询问：

SELECT Patient.patientname, AppointmentDateTime, Doctor.doctorname

FROM Appointment 

LEFT JOIN Doctor ON Appointment.doctorid = Doctor.doctorId  //have doctorId column common

LEFT JOIN Patient ON Appointment.PatientId = Patient.PatientId      //have patientid column common

WHERE Doctor.Doctorname LIKE 'varun%' // setting doctor name by using LIKE

AND Appointment.AppointmentDateTime BETWEEN '1/16/2001' AND '9/9/2014' //comparison b/w dates 

ORDER BY AppointmentDateTime ASC;  // getting data as ascending order

我编写了获得 date format like "mm/dd/yy" 的解决方案（以我的名字“VARUN TEJ REDDY”）

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SQL 中的多连接通过一个接一个地逐步创建派生表来工作。请参阅此链接解释该过程：

https://www.interfacett.com/blogs/multiple-joins-work-just-like-single-joins/

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