Question: I want to clarify difference between sequel vs SQL? Any one here know the difference between them?

Answer: 
in the early 1970s. Initially called SEQUEL (Structured English Query Language) and based on their original language called SQUARE (Specifying Queries As Relational Expressions).SEQUEL was later renamed to SQL by dropping the vowels, because SEQUEL was a trade mark registered by the Hawker Siddeley aircraft company.
I spent some time researching this old issue because it has come up *again* at my company. As John Woo said, it's a matter of *pronunciation*, not spelling. SQL is pronounced "sequel" by many, and there is some history there, beyond just trying to make an acronym out of an initialism. (NASA is an acronym; IBM is an initialism). 

But in writing, the best and most consistent way to refer to SQL Server or SQL databases, etc. is "SQL" --which takes the article "an" based on the initial letter "S". IBM and Microsoft style guides both agree on "an SQL " in writing, even if it might be pronounced "sequel" in some contexts. 

Our company was confused on this point for some years because a Google search shows much greater use of "...a SQL " in *English* hits. Also we have long relied on the Microsoft Manual of Style, and their "rule" was easily misinterpreted. They've updated it for the 4th edition--see pp 387-388. SO: In conversation, do what you want, as long as you understand that "sequel" works in English chat, but not necessarily for people whose native language is not English. **In writing, it's S-Q-L, and the article is "an".**