Functional and relational are 2 sides of the same coin. For instance:

x + y = z

Can be viewed as either a function called binary_addition with x & y as inputs and z as the output, or as the description of a relationship between 3 sets.

So, to solve the problem using SQL for example you would create a table binary_addition(x,y,z) and then fill it with all the true values that caused x + y = z to be true and then say "select z from binary_addition where x = 4 and y = 4"

In the functional model the computer stores the process for turning the input values you give it into the output values you want. In the relational model the computer stores a table containing all known possible input values, all known possible output values, and how they relate to each other, and gives you a way to retrieve them.

Note, the functional model is implemented by what that famous guy whose name I can't remember called "function-level" programming languages, not functional (aka lambda) languages.

The famous guy is named Robin Milner. He created the ML language. In lambda languages like lisp or javascript you can use functions as arguments and return values. I function-level, or applicative, languages you can only create new functions by combining existing functions. I think. I've never used a function-level language.