mysql - Subquery vs join -

select     id     posts     subject_id = 1      or subject_id in (         select related_subject_id          relatedsubjects         parent_subject_id = 1); 

trying select posts current subject sub-subjects stored in lookup table. above query works, wondering how accomplish same thing join

select distinct id posts p left join relatedsubjects r  on p.subject_id = r.related_subject_id , r.parent_subject_id = 1 p.subject_id = 1 or r.related_subject_id not null