Hello xue'er, "All for nothing" is not really an idiom ,it is just part of a sentence to express that everything pertaining to a certain subject under consideration is of no importance.It indicates that the final cause of an act or a state is insignificant, has no value or worth. The words are many times commercially used and there is a music band named "All for nothing". However there is another idiom " all or nothing". It literally means everything or nothing at all. Figuratively it refers to the best or highest time to choose to do something or not.

It means that you may have done a lot of work to achieve something but you did not get the result you wanted.

for me it's you do all the the best that you can do in work or in other things in the end it will be useless. cowboy...

It basically means: all that you've done, or that you have, has resulted in nothing. Eg, "I worked hard for ten years, and all for nothing!" This sometimes happens in English, where a longer and clearer sentence has been condensed into a short phrase.

It means a total waste of time and effort