merge different data set into single table

I have different data sets with different column length and different variable name as patient id. I want to save them in single table for statistical analysis with patient id on top without adding zeros. e.g. like this

    % code
  A B C
  1 3 4
  2 4 
  7

   P.S: I want to use final data in Classification learner which is not taking variables separately.