此查询需要花费大量时间，如何优化此查询？

问：

Select a, b, c, d, cast(replace(replace (stuff((
 select ', ' +LTrim(rtrim(e1)) 
 from (
     select e as e1, a, c 
     from table1 
     where a = am.a and c= am.c 
     group by a, c, e) as T 
  for xml path('')), 1,1,'c), '&lt;', '<') '&gt;', >') as varchar) as f 
from table1 am where c is not null