Is it possible to achieve a decisive victory when attacking a walled city?

[twgamer2020]

Specially if you attack the city by using only 1 army and the city also has an army inside with a full stack plus the garrison troops. In this case you are slightly outnumbered.
Is in this case almost impossible to achieve a decisive victory or even a close victory if the battle is fought with the enemy inside the city? Is a pyrrhic victory the most likely result in the case you win?
I know it is better to first besiege the city for several turns to make the enemy army sally out. But I'd do that only if I have the time to wait for more turns. Sometimes you can't wait for several turns if you are in a hurry ( imagine there is one or two enemy armies on the way to the city and that they'll reach the city within 2 turns ). If they reach the city, you'd be in big trouble because you'll be greatly outnumbered ( like 3:1 ). And there can be more reasons why you can't wait for several turns.

EDIT:
I was referring to Rome 2 Total War but maybe the tips ( on how to fight the battle in the scenario I mention ) you suggest me can also be applied in Total War Attila