SQL 先Group by 分组后 left join 语句

select t2.dep_no,t2.dep_name,t1,empCount,t2.empAvgAge
from(
     select t.dep_no, count(t.emp_no) as empCount, avg(t.emp_age) empAvgAge,
     from employee t
     where t.emp_no < 80707999
     group by t.dep_no
)t1
left join department t2 on t1.dep_no=t2.dep_no