Question

I have a table Action with a (part of the) structure like this:

Action:
ActionTypeID,
ActionConnectionID
…

ActionTypeID refers to types 1-5 which correspond to different tables (1=Pro, 2=Project etc.)
ActionConnectionID is the primaryKey in the corresponding table;

ie. ActionTypeID=1,
ActionConnectionID=43 -> would point
to Pro.ProID=43 and ActionTypeID=2,
ActionConnectionID=233 -> would point
to Project.ProjectID=233

Is there a way to ‘dynamically join the different tables depending on the value in the ActionTypeID column?
ie. for records with the ActionTypeID=1 this would be:

Select Action.* 
From Action Left Join Pro On Action.ActionConnectionID=Pro.ProID

for records with the ActionTypeID=2 this would be:

Select Action.* 
From Action Left Join Project On Action.ActionConnectionID=Project.ProjectID

etc.

If this is not possible to accomplish in one query I will have to loop over all the possible ActionTypes and perform the query and then afterwards join the data in one query again – that would be possible, but doesnt sound like the most efficient way 🙂

Answer 1

Something like this should do:

Select Action.*  
From   Action 
       Left Join Pro 
             ON Action.ActionConnectionID=Pro.ProID and ActionTypeID=1
       Left Join Project 
             ON Action.ActionConnectionID=Project.ProjectID and ActionTypeID=2

If that doesn’t work for either try using dynamic sql which is a bad solution or properly normalize your data.