Identifying values that are not existent in other table I've got two tables that have one to many associations on a pmid.  So if one table has an pmid, the second table should have multiple rows with the same pmid.  However, something went sideways and I'm missing my latest batch of pmids in the second table.  These queries, should help illustrate the problem, but I can't figure out how to get the ids from the first table that are actually missing in the second table.

select count(*) from abstract_mesh am; #2167101

select count(*) from abstract_mesh am
join abstracts a on am.pmid = a.pmid; #2133848

select 2167101 - 2133848;  #33253

select count(*) from abstract_mesh where pmid is NULL; #33253


So as you can see there are 33,253 rows in abstract_mesh that have no pmids.  I simply want to identify which pmids I should be interested in from the abstracts table.
These are pmid values that are in abstracts but not in abstract_mesh

select a.pmid from abstracts a
left join abstract_mesh am on a.pmid = am.pmid
where am.pmid is null;


or

select a.pmid from abstracts a
left join abstract_mesh am using (pmid)
where am.pmid is null;