Being a professional, I always leave my work/task in-between and at
once look into their queries.
That's not being professional. That's just being accommodating - and in this case perhaps overly-so.
Sometimes I am doing very important tasks which require concentration,
but I still can't find a way to say 'NO' to their queries.
How to handle interruptions/distractions by others at work?
I suspect you already know this, but you simply have to learn to say "No" when it is warranted.
If you truly have very important tasks, you can either say something like "No, I can't help you this time, I have to concentrate on the task I have." or "No, I can't help you right now. Perhaps later, once I have finished my task."
Sometimes, you can use your boss as the excuse - "Sorry, [boss] wants me to get this task done, so I can't help you. If you think it's important enough, we could go to [boss] and ask if things should be re-prioritized."
Depending on personalities, experience, and the work hierarchy, it can often be uncomfortable turning down a request for help. Additionally we sometimes unconsciously want to be known in the office as "the guy or gal who helps everyone". But if doing so gets in the way of completing your tasks, then you are shortchanging both yourself and the company.
Try saying "No" a few times. It will be hard at first, but you'll learn that it gets easier with time. And you'll likely also find that your colleagues will understand.