Just trigger in my mind when I was going through some websites were they having upper case and lower case combination in url something like http://www.domain.com/Home/Article
Now as I know we should always use lowercase in url but have not idea about technical reason. I would like to learn from you expert to clear this concept why to use lowercase in url. What are the advantages and disadvantages for upper case url.
The domain part is not case sensitive. GoOgLe.CoM works. You can add uppercase as you like, but normally there's not a reason to do so and, as stated in the comments below, may hurt your SEO ranking.
The path part is or is not case sensitive, depending on the server environment and server. Typically Windows machines are case insensitive, while Linux machines are case sensitive. This means that you should stick to lowercase or you risk introducing a bug that's really hard to hunt down (mismatched case that doesn't matter on the dev server).
The query string part is available to the server as it is. You can readily use mixed-case as you like, or discard the case (toLowerCase(...)). This also means that using a base64-encoded keys will work. You can't expect the users to type that correctly, though.
The hash part (called "fragment identifier") is only available to the client code, not to the server. Javascript may distinguish between the cases as it likes, and so does the browser. url#a will scroll to the element with the ID a, but url#A won't.