Go to www.w3schools.com and learn SQL with examples
or click on this below link to download a pdf file from which you can learn SQL in21 days.
http://www.fdi.ucm.es/profesor/albertoe/grupodesarrollo/sql/material/%28ebook%20pdf%29%20teach%20yourself%20sql%20in%2021%20days.pdf
I've only tried learning it one way, and that way worked very beautifully. It's in w3 schools. Check this link:-
http://www.w3schools.com/sql/default.asp
Everything in that tutorial is very basic, and they structure the difficulty of each section so you wouldn't have to move to unnecessary advanced stuff.