两张表相同字段求和的SQL

两张表相同字段求和的SQL？

select coalesce(a.name,b.name), coalesce(a.dm,b.dm), sum(isnull(a.age b.age,0))from a full join b on a.name=b.name and a.dm=b.dmgroup by coalesce(a.name,b.name), coalesce(a.dm,b.dm)