What is considered as win a combat?

You win a combat if you are the Attacker or Defender and you are the only player with units still at that battlefield when combat result is determined (Rule 466.3.a).

Damage doesn't have to happen — you win even if the opponent's units were bounced, killed, or moved away during the Showdown, as long as only your units remain. Three exceptions produce "No Result" where nobody wins: attackers were Recalled during Combat Cleanup (even if only defenders remain), both players still have units, or neither player has units. Also note conquering an empty battlefield is not winning a combat — no combat occurred (FAQ #9092).