如何根据表格问题计算表格答案中的正确答案行？

Question

how to count row from t_answer table that has the correct answer according to t_questions ?,

t_user
   id (int 10) PK
   email (varchar 191)
   password (varchar 191)
   name (varchar 191)

t_question
   id (int 10) PK
   question (varchar 191)
   choice_a (varchar 191)
   choice_b (varchar 191)
   choice_c (varchar 191)
   choice_d (varchar 191)
   correct_answer (enum 'A','B','C','D')

t_answer
   id (int 10) PK
   id_question (int 10) FK
   id_user (int 10) FK
   user_answer (enum 'A','B','C','D')

I have tried it in the following way,

$id_user = $request->input('id_user');

$correct_answer = DB::table('t_answer')
    ->join('t_question', 't_answer.id_question', '=', 't_question.id')
    ->where('t_answer.id_user', '=', $id_user)
    ->where('t_answer.user_answer', '=', 't_question.correct_answer')
    ->count();

but the result is always '0' even though there is a correct answer