For me, I generally mix my sets using BPMs because they flow well together. Mixing songs in the same key is secondary because a lot of times I go off of feel and what sounds best as opposed to being locked into the “Camelot wheel”.
That said, key mixing has it’s place. Moving up and down the tempo is proven to be effective, as many DJs that I have seen will do that. However, a word of caution is not to fall into the “Tempo trap”. That means as you continue to expand your skills, find ways to vary your song selection as far as tempo. As a DJ you don’t want to be predictable, and mix the same songs in the same order all of the time.
This website has articles that have provided great tips in that regard. I’m not the most technology savvy person, so I’m not gonna provide a link. But I have an article archived from April 29, 2014 last year from DDJT that has helped me immensely: “7 Insider Tips For Making Exciting Tempo Changes While DJing”. Some of the ones that I regularly use and some not mentioned there are using the echo, kill the music completely (silence), drop a song on the “1” beat”, the 50 % rule, etc.
To your last question, if I’m understanding it right, you can use your DJ program to order the songs by BPM, Key, or however you’d like. Serato DJ, Virtual DJ, Traktor should all be be able to do that. Serato (which I use) doesn’t detect the keys of songs, but you can download any # of Key detection software programs that can do that (There are some free ones i.e. KeyFinder).
Hope this helps!
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