an analogy: you need to gather all keys you have, you know the keys are in your house but don't know exactly where each and every one of them at;

you have 2 ways to find them, 1st is to search every single corner in and out manually by your hands&eyes until you found all of the keys, 2nd is to use a special device that can identify key objects and locate the exact positions of every key;

keys = comments

house = postIds

1st way = select from where

2nd way = select from where in

would you say this analogy matches what you want to express?