The best way to learn SQL is sit down and view tutorials on Microsoft's website.
Microsoft has some SQL Tutorials that can help you learn SQL and many people have learned a lot about SQL through Microsoft.
Taking a college course in SQL is also a good idea especially if you want to learn SQL to work for a company needing someone who knows about SQL.
You can also learn SQL by installing SQL on a server and playing around with it from your home and learn it.
You can also learn coding and learn to manage and operate, update etc servers and work with Windows Servers or Linux servers.
However the best way to learn SQL is to take an SQL class and study hard.
Also codeacademy.com is another good website to learn SQL.