title: sql site or book with exercise queries more complicated than www3.schools.com
body: <p>Hi
  is there any online resource for practicing SQL queries more complicated ?
  basically I had a db dev until now and I kind of forgot. I want to switch jobs and freshen up a little. 
  They should more complicated than 2 joins or something, as my level is medium or senior hopefully :)
  Thank you</p>