I have gone through a bunch of SO questions for the correct way to use the mySQL's `GREATEST()`function but none of them show how to use it with a column.
[Official Documentation](http://dev.mysql.com/doc/refman/5.6/en/comparison-operators.html#function_greatest) gives for a list of manually entered list.

[This](https://stackoverflow.com/questions/1632881/how-to-get-the-greatest-of-two-columns-values-in-mysql/1632940#1632940) is about two values only.

I was also unable to get my answer in [A SO Search](https://stackoverflow.com/search?q=greatest+mysql+%5Bmysql%5D).

Regards,