[TheNakedEmperor]

I want to study SQL, can someone link me to a good ebook/site in which I can learn it?

also, how long will it take me approx to learn it?

[ExplodingKnees]

w3schools.
If you actually learn it, 3 hours.

[How bout using your brain]

If you actually learn it, 3 hours.

lol there is more to SQL than select, union and insert....

[Dr. Gonzo]

w3schools.
If you actually learn it, 3 hours.

3 hours, will give you a basic knowledge of SQL maybe. By no means will it give you complete mastery though.

[Mr. Corleone]

If you want to learn SQL specifically for hacking, i'd recommend hacking challenges that include SQL knowledge.
I believe there was some SQL dude that got like 50+ challenges for that, you can look it up, it's easy to find I think.

[ExplodingKnees]

lol there is more to SQL than select, union and insert....

What, UPDATE, SET, DELETE? They're all queries with input and output. There's join and sub-queries, there's table types, functions, there's operands and operators, parameters and such. It can get a bit more complicated, but not a lot. I can't give more than a day or two of time dedication to learn SQL. It's not that difficult, on par with HTML.