多个leftjoin重复计数

各位大神，今天见到一个sql，如下，查询t1.id有在与t2.id进行左联结时的条数，发现运用了大量重复的左联结，感觉很不好，但又不知道怎么修改这个sql，请指教：

create table t1(id number);
create table t2(id number,val varchar2(10));

select a1.id,count(1)
  from t1 a1
  left outer join t2 b1
    on a1.id = b1.id
   and b1.val = 'a'
   left outer join t2 b2
   on a1.id=b2.id
   and b2.val='b'
   left outer join t2 b2
   on a1.id=b2.id
   and b2.val='c'
   left outer join t2 b2
   on a1.id=b2.id
   and b2.val='d'
   left outer join t2 b2
   on a1.id=b2.id
   and b2.val='e'
group by a1.id;