Right.

The website is not just clear MySQL, it has some logic written in PHP. Which should check if "mandatory" is off, "unique" is on, then if field is empty or not, also by trimming spaces - and if it's empty - to not do anything. If not empty - then check if entered value is unique by db table.

If I want users to be able to leave the field empty, I should uncheck "mandatory". 

If I want them to fill it with any data, I should check the "mandatory".

Simple. It works like this on all platforms.