Is it something particular or is it something more general?
Typically, business rules are of the form "if(condition) then action", while business logic tends to describe a larger set or a sequence of both business rules and other logic. So "business logic" is more general any code that implements logic specific to your problem domain, where "business rule" is a specific concept.
Examples (pseudo code):
// business rule
if(sales-revenue > 1000) then send("thank you")
//business logic
sales-revenue = sum(all items in order)
bonus-points = sales-revenue * .1
executeBusinessRules()
Do I need to learn UML?
No, you don't have to learn UML. But it may help to visualize your solution, and in fact make it easier to describe because you have to think it through more thoroughly - if you can't draw it in UML, probably something is not quite right yet.
Business rules can be nicely described with state charts or activity diagrams, and business logic in general can be described in sequence diagrams or activity diagrams.
Does the fact that I use MVC affects the way I'll describe it?
It shouldn't - if it does, this may indicate poor separation of concerns. MVC is a way to organize your code, or more specifically to keep aspects of your implementation separate (aka 'separation of concerns').
The M(odel) in MVC is what should implement your business logic.
Best Answer
One way to do this is by creating a Unique Index on the column.
There should be a way to display AKn of the left hand column of the design surface, but I don't know how to do this.