what's the different among "at which point/time ,by which point/time and in which case"
i guess we can use all of them at the beginning of a non-defining relative clause,but it is a little hard for me to make a appropriate choice among them.
for instance,this sentece,Sam started to tell one of his terriblr old jokes, （）i decided that i should go home.
i tend to use in which case but the correct answer is at which point.i feel really confused about that.

Question

what's the different among "at which point/time ,by which point/time and in which case"
i guess we can use all of them at the beginning of a  non-defining relative clause,but it is a little hard  for me to make a appropriate choice among them.
for instance,this sentece,Sam started to tell one of his terriblr old jokes,  （）i decided that i should go home.
i tend to use in which case but the correct answer is at which point.i feel really confused about that.

Answer

In this context:
at which point ... = and that was the moment when ...

Use "in which case" like this: 
"You have decided to travel by bus instead of train, in which case you will need more time (because buses are slower)"

This is about the consequences of choosing options.  It is not about time.

Answer

The sentence still gives complete meaning even you don't add relative pronoun. It means the sentence is contact claus.