require 'pg'
db = PG.connect dbname: 'josh_testing'
db.exec(<<~SQL).to_a
  drop table if exists lefts cascade;
  drop table if exists rights cascade;
  create table lefts (
    id serial primary key,
    val text
  );
  create table rights(
    id serial primary key,
    left_id integer references lefts (id),
    val text,
    is_relevant boolean
  );
  insert into lefts (val) values ('la'), ('lb'), ('lc');
  insert into rights (left_id, val, is_relevant)
    values (1, 'ra', true), (2, 'rb', false);
SQL

# With the condition (is_relevant) on the join clause
db.exec(<<~SQL).to_a
  select lefts.id lid, rights.id rid, lefts.val lval, rights.val rval
  from lefts
  left outer join rights on lefts.id = rights.left_id and is_relevant
SQL
# => [{"lid"=>"1", "rid"=>"1", "lval"=>"la", "rval"=>"ra"},
#     {"lid"=>"2", "rid"=>nil, "lval"=>"lb", "rval"=>nil},
#     {"lid"=>"3", "rid"=>nil, "lval"=>"lc", "rval"=>nil}]

# With the condition in a where clause
db.exec(<<~SQL).to_a
  select lefts.id lid, rights.id rid, lefts.val lval, rights.val rval
  from lefts
  left outer join rights on lefts.id = rights.left_id
  where is_relevant
SQL
# => [{"lid"=>"1", "rid"=>"1", "lval"=>"la", "rval"=>"ra"}]

# !> NOTICE:  drop cascades to constraint rights_left_id_fkey on table rights