Can you have a complex sentence for whereas?

Absolutely. In fact, a complex sentence is the sine qua none condition under which "whereas" can be used.

Whereas can never be used on its own, except for purely stylistic matters, and would be in this case irrelevant without a given situation. Thus, "whereas" will always be found in complex structures.

Also, the following example should make you understand better how "whereas" transform and independent clause into a subordinate one, whilst showing you that whereas can be found in a simple sentence, but it will always be for stylish matters.

"Whereas Porthos always comes in search." (James Matthew Barrie, THE LITTLE WHITE BIRD). This example is a full sentence, but makes no sense without the previous one (that was "The manners of Porthos are therefore prettier than the manners of David, who when he has sent me to hide from him behind a tree sometimes comes not in search, and on emerging tamely from my concealment I find him playing other games entirely forgetful of my existence"). This given example, in fact, becomes a fragment of a sentence with the use of "whereas". Indeed, "Porthos always comes in search" would be a valid, solid sentence, and would not depend on the previous one to exist alone.