https://www.codeschool.com/search?utf8=%E2%9C%93&loc=hero&query=sql
Code school is a really good introduction to most concepts regarding SQL, but it is only really possible to have a really good understanding if you truly start with the basics so I would suggest starting here and then spending quite a few hours just messing around with an sql database of your own.
Best of luck to you, hope this helps
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solved Understanding SQL in depth [closed]