No. Entering a building is not a sin (unless you are under age and breaking the law). Having a drink isnt a sin either. Getting drunk and loosing your self control is a sin.
That said, God says we should strive to be good witnesses, living our lives so that others can see our Him in our lives. If you are seen entering a bar by people who know you claim to be a Christian then that will ruin your witness to them and however many people they tell 'hey you know who i saw going into/coming out of a bar?'.
The Bible say that if you feel wrong about doing something and then you do it, it's a sin. there's nothing wrong with going in a bar, but if you're a recovering alcoholic or something, then it would be bad
But, but Proverbs 6:27-28 says, "Can a man take fire in his bosom, and his clothes not be burned? Can one go upon hot coals, and his feet not be burned?" In other words, you shouldn't put yourself in a position where there is temptation because you'll more than likely fall into it.
It really depends on the place. Many restaurants have bars in them. What is the real intent of the business and the people there? The Bible declares that light has nothing to do with darkness. We should come out and be separate from the world. We should be holy and live holy lives - lives that glorify Jesus. If we love Jesus, we should hate sin.