MySQL：如何以多对多关系在两个表之间创建映射

Question

I have two relevant tables:

Trainee and Device

Each trainee needs to do a training with the device, in order to be allowed to use it. As a trainee needs to be trained in multiple devices and one single device can be used by multiple users I have created a many-to-many relationship between those two.

MySQLWorkbench therefore created me another table which is called 'trainee_has_device' and holds a foreign key to trainee.Id and a foreign key to device.Id.

So far, so good.

I need to get the training status for a trainee to a specific device, so I added a property TrainingStatus on 'trainee_has_device'. My tables then look like this:

|trainee|             
--------------------- 
id 
surname

---

|device|
---------------------
id
deviceName

---

|trainee_has_device|
---------------------
traineeId
deviceId
TrainingStatus

I need to get the training value of every person to every device even if there is no entry in 'trainee_has_device' (if so I would like to get null)

My query looked like this:

SELECT * FROM trainee LEFT OUTER JOIN trainee_has_device ON trainee_has_device.traineeId = trainee.id LEFT OUTER JOIN device ON device.id = trainee_has_device.deviceId

But Unfortunately I did only get entries which were listed in trainee_has_device or null.

How can I get the TrainingStatus of every person to every device even if there is no entry in 'trainee_has_device'?

Answer 1

I need to get the training value of every person to every device even if there is no entry in 'trainee_has_device'

This doesn't make sense, the trainee_has_device table has to be filled first or it won't show you any data. In real world, all of the trainees has to be assigned a set of devices with a default status (New,Familiar...) for each assignment. So if the connection isn't available between Trainee A and Device C it just means that Device C isn't assigned to Trainee A

Answer 2

I managed to get it working with a subquery:

SELECT trainee.id as TraineeId, trainee.Prename, trainee.Surname, trainee.Mail, trainee.Department,
trainee.Telephone, device.Name as DeviceName, (SELECT IF (trainee_has_device.trainee_id = trainee.id AND
trainee_has_device.device_id = device.id, trainee_has_device.TrainingStatus, 'N/A')
FROM trainee_has_device WHERE trainee_has_device.trainee_id = trainee.id AND device.id = trainee_has_device.device_id) AS TrainingStatus
FROM trainee JOIN device ORDER BY trainee.Surname, trainee.id, device.Name ASC