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

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

I think this depends also on siege engines - a lot of the issues your army will face will be related to the walls and the garrison around it, since it will fire upon you. If you manage to defeat that part then you should be good to go because it will give you a numerical advantage in terms of soldiers overall.

I think also you're better off just having 2 armies instead - one with siege engines, one with your regular army.

When attacking a walled city, I'd recommend using archers instead of slingers because with slingers you can't fire at any enemy unit that is inside the city. On the other hand, archers shoot over the walls