left join A and B may still lose obs in A

Imaging data set A has the demog information, and data set B has multiple outcome variable and covariate information. B has less subjects than A. To create data set C which is merged from A and B and contains all subjects in A (even the subject is not in B), the following sql statement would be fine

proc sql,
create table c (drop=subjid) as;
select a.*, b.* from a left join b (rename=(pat_id=subjid))
on a.pat_id=b.subjid;
quit;

However, since B has multiple outcome variables, there can be an intent to filter out one outcome variable like the following

proc sql,
create table c (drop=subjid) as;
select a.*, b.* from a left join b (rename=(pat_id=subjid))
on a.pat_id=b.subjid
where b.outcomeName='outcome A';
quit;

This is wrong, because the field of outcomeName will be missing (and filtered out) for subjects in A but not in B. The solution can be either to filter in the separate data step, or the following sql statement

proc sql,
create table c (drop=subjid) as;
select a.*, b.* from a left join b (rename=(pat_id=subjid) where=(outcomeName='outcome A'))
on a.pat_id=b.subjid;

quit;