根据列值有条件地联接到表

Question

We have the following table which contains deviceId and customerPrefix columns.

tblDeviceInfo: 
deviceId       customerPrefix
1             ABC
2             XYZ

Based on the customer prefix we also have device tables, I have listed only 2 below but we have too many of them and the above tblDeviceInfo table can contain many different customerPrefixes, I know this is a bad practice but this is what we have at the moment and we have to work with it for the time being.

tblDeviceABC
deviceId           deviceName
1              NameABC1
2              NameABC2

tblDeviceXYZ
deviceid           deviceName
1             NameXYZ1
2             NameXYZ2

When I am querying the tblDeviceInfo (which can contain quite a few rows) I would like to be able to get the following:

deviceId, deviceName, customerPrefix
1            NameABC1          ABC
2            NameXYZ1          XYZ

Currently we get everything from the tblDeviceInfo table, start looping (in C# app) and get the device details from the relevant table based on the customer prefix, I was wondering if there is a way of joining to a table based on the column value and get everything back with one sql like this with dynamic sql

    @sql = 'Select deviceId, customerPrefix From tblDeviceInfo i
  INNER JOIN tblDevice' + d.customerPrefix + ' d ON d.deviceId = i.deviceId'

This is not quite right but is there anyway of achieving this with dynamic sql or with any other way?

Thanks

Answer 1

It's not just bad practice, it is awful.

But, you might be able to solve the query something like this:

Select deviceId, customerPrefix From tblDeviceInfo i
LEFT OUTER JOIN tblDeviceABC dABC ON dABC.deviceId = i.deviceId
...
LEFT OUTER JOIN tblDeviceXYZ dXYZ ON dXYZ.deviceId = i.deviceId

There will be a lot of null checking on the result, but you should be able to make it in one (expensive) query.

Answer 2

You can union all the tblDEVICE in a subquery andjoin it with the infor table,

SELECT  b.*, a.customerPrefix
FROM    tblDeviceInfo a
        INNER JOIN
        (
            SELECT deviceID, deviceName From tblDeviceABC
            UNION
            SELECT deviceID, deviceName From tblDeviceXYZ
        ) b ON a.deviceID = b.deviceID