MySQL多个联接一个查询？

我有以下查询：

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

所以我正在使用INNER JOIN并抓住image_id。所以现在，我要获取该image_id并将其images.filename从图像表中转换成。

如何将其添加到查询中？

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，由于它是一个INNER JOIN，如果您的消息中没有图像，则将跳过整个行。如果可能的话，您可能需要执行一次操作LEFT OUTER JOIN，该操作将仅在存在的情况下返回所有仪表板消息和一个image_filename（否则，您将获得null）

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 

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

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）患者组成列PatientID，PatientName

表2）约会包括列AppointmentID，AppointmentDateTime，PatientID，DoctorID

表3）Doctor包含DoctorID，DoctorName列

查询：

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

我写了解决方案以获取日期格式，例如“ mm / dd / yy”（以我的名字“ VARUN TEJ REDDY”命名）

SQL中的多重联接通过一个接一个的逐步创建派生表来进行。请参阅此链接以说明过程：

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