I'm exploring rawlocation.csv and location.csv, I use stata trying to merge them two. I found rawlcoation location_id and location id are many to 1 relationship. I use merge m:1, but neither _merge==1 and _merge==2, which means rawlocation has unique observations that not appear in location, location also has unique observations that not appear in rawlocation dataset. I wonder why this is the case? Shouldn't _merge==1 or ==2 be 0? Maybe this is due to the disambiguity?