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What is the idiom or proverb or phrase for this "hard packing but loose knot"?

For example, you took hard preparation for the exam, but, didn't attend it.

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You could say that it was all for naught:

without achievement or result

I spent a long time preparing for my test, but it was all for naught.

[The Free Dictionary]

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    Or, "All for nothing" :) – NVZ Apr 22 '16 at 15:16
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Not exactly what you're asking, but you could say that all of your hard work was in vain.

http://idioms.thefreedictionary.com/in+vain

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