How far to go with database constraints?

Question

This question is related to another question I asked. In my other question I ask peoples opinions about 3 different ways I could construct a database. The cleanest way I ca

Answer 1

Why not have:

Alert [Table]
- Id [PK]  
- SensorReadingId [FK]  
- LoggerReadingId [FK]  

And then you fill either the SensorReadingId or the LoggerReadingId. I suppose your structure is a simplified one, but often a table with nothing else then one PK is redundent.