Hi guys I dont know how to play piano so I wanted to know, Can I be play without restrictions if I only learn to play in one key, and use the transpose if I have to. Would I be able to play like Rousseau, Jarrod Radnich e.t.c if I played in one key? Thanks
Yes, and many people do it. Particularly songs, which may need transposing into other keys to suit a singer's vocal range.
HOWEVER: this does NOT mean every piece will use only diatonic notes. That means , in key C, exclusively using the white keys. Lots of tunes 'in a key' will use chromatics (notes not included in the standard scale) so, keeping in key C, the black keys can come into play.
So, the answer is yes. You can learn any piece in any one key, using all the notes applicable to that piece, hit the transpose button, and be in whatever key you wish. Be aware that it will only sound good in 12tet, which is the tuning most of us use on keyboards these days. Obviously, with an acoustic piano, there's no transpose button!
Can I be play without restrictions if I only learn to play in one key
Well, no. You'd be restricted to the song you are learning - that's assuming you had a way to learn it without being able to read sheet music, which I'd argue is not something that one can learn selectively (or even if they could, would save any time doing to properly understand it)
Suppose you were to learn the songs "by ear" in your one key, well, in this case you haven't learned to play the piano in one key at all, you've learned how to reproduce a single title using whatever self-learning cognition and key-unaware process you adopt.
Even the more generalized and common interpretation of "learning to play in one key" - i.e. using the white notes and major/minor chords based on C and using the transpose button - still isn't playing without restriction, by virtue of the fact that playing in any key without restriction would mean understanding it's modes.
Any major song can be played in C major. I asked about this too (when I was just a young grasshopper) and it was very significant. And it's a great way of learning. Because then you just use C major as your "reference" key and you can transpose it and use the same relative notes and chords the same way in any other key. It's a great technique for playing by ear and developing theory.
'Jazz Tutorial' made a video about this called Fixed Key Ear Training and he mentions that "this is probably the most important lesson I will ever make on music.." in the start.