A Question About an MLB Rule...?

Let's say that a SP pitched to the 6th and was down 1-0,and the relief pitcher comes in next inning and the team goes up in their at bat then the relief pitcher gets the win.It's all about who was pitching when they were up the final time.A SP can pitch to the 4th and get the win as long as they were up the rest of the game.It's just the way it is.Now let's say the NL took the lead and held it from the beginning of the game then Roy Halladay (I think that's how you spell his name) would've gotten the win regardless of how long he was in there.