We may compare those commands to SQL statements: Select prepares the "where" conditions, filters based on any fields from the Table/View. Fetch Next defines the "select"ed fields, for each record returned.
Now Fetch Single does both operations as one:
Fields from the selected index become conditions - equivalent to a Select command;
Any other fields are the returned data - equivalent to a Fetch Next.
Actually if you use fetch single, without passing a key input parameter it won't allow you to save and exit. so that there is no question of select and fetch single. you have to pass the key and fetch the record using fetch single only. You can reach me out at email@example.com