If SQL cell has value, default selected item in combobox equals the value in that row

[CoStratify]

Sorry my subject isn't very clear. 

To be a bit more clear...

I have a combo box that is looking up the AppName from a SQL table. The idea of selecting the AppName is that it will write that App's AppID to the column dependant on what has been selected.

The area I would appreciate help with is... I want the Combobox's defaultSelectedItem to show if the row already has an AppID assigned and therefore automatically shows the related AppName on the combobox. 

Please could you advise what I need to put in the DefaultSelectedItem box it would be greatly appreciated, as I'm truly stuck haha!

Thanks!

[CoStratify · Accepted solution]

I think I've worked it out by using a dropdown instead of a combobox.

I have put the following in the default section.

LookUp('Datasource', ThisItem.AppID = AppID, AppName)