Resolved: proc sql left join based on condition with missing values

    data a;
     subj=1; dat="01jan2022"d; output;
     subj=1; dat="01feb2022"d; output;
     subj=1; dat="05mar2022"d; output;
     subj=2; dat="10may2022"d; output;
     subj=2; dat="11jun2022"d; output;
    run;
    
    data b;
     subj=1; dat_new="01jan2022"d; other_var=1; output;
     subj=1; dat_new="01feb2022"d; other_var=2; output;
     subj=1; dat_new="05mar2022"d; other_var=3; output;
     subj=2; dat_new=.; other_var=11; output;
     subj=2; dat_new="11jun2022"d; other_var=21; output;
    run;
    
    proc sql;
    create table ab_reconciliation
    as
    select a.subj, a.dat format=date9., b.dat_new format=date9., b.other_var
    from a as t1
    left join b as t2
    on t1.subj=t2.subj
    and dat=dat_new; 
    quit;

data a;
  input subj dat :date.;
  format dat date9.;
cards;
 1 01JAN2022
 1 01FEB2022
 1 05MAR2022
 2 10MAY2022
 2 11JUN2022
;

data b;
  input subj dat_new :date. other_var;
  format dat_new date9.;
cards;
 1 01JAN2022 1
 1 01FEB2022 2
 1 05MAR2022 3
 2         . 11
 2 11JUN2022 21
;

data want;
  set b(in=inb rename=(dat_new=dat)) a(in=ina) ;
  by subj dat;
  if first.id then other=.;
  if inb then other=other_var;
  retain other;
  if ina ;
  drop other_var ;
  rename other=other_var;
run;