为什么左联接不起作用而右联接起作用？

Question

i have 3 tables,

Transport.Devices, Transport.Conductors, Transport.ConductorDevices,

i am applying LeftJoin on Devices table so it should also display those Devices which are not assigned, but it doesn't work , why ? i tried Right join and it worked but left doesn't work.

Select Transport.ConductorDevices.ID, Transport.ConductorDevices.Device_ID,Transport.ConductorDevices.Conductor_ID,
        Transport.Conductors.Name as Conductor, Transport.Devices.TerminalSNO as Terminal
        from Transport.ConductorDevices
        Inner Join Transport.Devices
        ON Transport.Devices.DeviceID=Transport.ConductorDevices.Device_ID
        left Join Transport.Conductors
        ON Transport.Conductors.ConductorID= Transport.ConductorDevices.Conductor_ID

Answer 1

A left join != right join, a left join does include all on the left and the right join includes all on the right. In this case you need all on the right. That is why the right join is working for you.

sql_join_left

Answer 2

If you need all devices (not assigned too) you can start FROM with devices:

 from Transport.Devices
    LEFT JOIN Transport.ConductorDevices
    ON Transport.Devices.DeviceID=Transport.ConductorDevices.Device_ID
    LEFT JOIN Transport.Conductors
    ON Transport.Conductors.ConductorID= Transport.ConductorDevices.Conductor_ID