Confidence score function mysql implementation (with negative score when downvotes > upvotes)

confidence.sql

/* confidence score */
/* http://www.evanmiller.org/how-not-to-sort-by-average-rating.html */
drop function if exists confidence_score;       
delimiter $$
create function confidence_score(upvotes int, downvotes int) returns double
       begin
              declare n      int;    
              declare z      double; 
              declare p      double; 
              declare l      double; 
              declare r      double; 
              declare u      double;
              declare t_up   int;
              declare t_down int;
              declare sign   int;

              /* reverse and make negative if more downvotes */
              if downvotes > upvotes then
                     set t_up      = downvotes;
                     set t_down    = upvotes; 
                     set upvotes   = t_up;
                     set downvotes = t_down;
                     set sign      = -1;
              else
                     set sign      = 1;      
              end if;       

              /* equation I don't understand */
              set n = upvotes + downvotes;
              set z = 1.96;
              set p = upvotes / n;
              set l = p + 1 / (2 * n) * z * z;
              set r = z * sqrt(p * (1 - p) / n + z * z / (4 * n * n));
              set u = 1 + 1 / n * z * z;

              if n = 0 then return 0; 
              else return sign * (l - r) / u;
              end if;

       end $$
delimiter ;