Interface is currently down.
This service has been running for around 20 years now, and I stopped teaching
database systems about 15 years ago. The day has finally came where I think
I should start to close down the site. I will leave the static content up for
now, but no doubt the whole site will close in the near future.
For an alternative, consider sqlzoo.net. This was written by a colleague of mine, and offers some great features for learning SQL.
I hope you have enjoyed the site over the years...