from where i can download sql?

I think Oracle is an commercial DB software. I think there is some free but reduced edition of it for Linux(Not sure about this though)
If you want to learn SQL, you can use MySQL(free, open source,), SQLite, PgSQL, MS-SQL, etc.l

You mean the Express Edition?

*www.oracle.com/technology/products/database/xe/index.html

My Personal Recommendation will be MySQL or PostgreSQL though.

Get oracle, and the sql developer. Both are available free of charge from oracle.com

I used it myself

You in 12th ? Then you hardly need practice with Oracle. Just learn from the theory book.

All you need to remember to avoid getting pwned is

1. the order of the few commands which you are tested in,
2. that the date is expressed as 31-jan-08 and not as 31/1/2008,
3. that you need to always name constraints,
4. that you need to end statements with a semi-colon (,
5. that you need to specify the no. of digits for a number (which is declared as Item Number(5) where item is the object name and number is the type of variable and 5 is the capacity)
6. That you need to specify size of characters and need to use varchar2 preferably as character variable
7. That characters and dates are to be entered in quotes
8. That you type what you learn in a book and hence there is nothing to worry about

^ There is no replacement for practical knowledge...Even a 15min practical experience would go a long way in being well versed with SQL in Class 12...and would in fact help in remembering after years.