在一个查询MySQL多重连接？
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 所以我使用INNER JOIN并抓取image_id 。 所以现在，我想把这个image_id，并把它从图像表变成images.filename 。
我怎样才能添加到我的查询？
你可以简单地添加这样的另一个连接：
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，因为它是一个INNER JOIN ，所以如果你有一个没有图像的消息，那么整行将被跳过。 如果这是可能的，你可能想做一个LEFT OUTER JOIN ，它只会返回所有的仪表板消息和一个image_filename（否则你将得到一个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
只需添加另一个连接：
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 JOIN的经验。
我有3个表格：
表1）患者由列PatientID，PatientName组成
表2）约会包括列AppointmentID，AppointmentDateTime，PatientID，DoctorID
表3）医生包括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scheme来获取date格式，如“mm / dd / yy” （以我的名字“VARUN TEJ REDDY”）
