Subtraction in the Same Column using Proc SQL

[mkeintz]

If the data are sorted by subject  (but can have any order within subject), then the DATA step solution is the most tempting:

data have;
input Subject  & $15.   Office & $15.  Date   Value ;
cards;
Patient 1         Visit 1            12       20  
Patient 1         Followup 2          4       35  
Patient 1         Followup 2          2       54  
Patient 1         Followup 3          3       29  
Patient 1         Followup 3         11       13  
Patient 1         Followup 3          5       98  
;

data want (drop=base_value);
  merge have (keep=subject office value where=(office='Visit 1') rename=(value=base_value))
        have;
  by subject;
  if office='Visit 1' then output=value;
  else output=value-base_value;
run;

This assumes that each subject has exactly one "Visit 1" record.