Question

First query gets the ID and registration time:

SELECT
  t1.mid
  t1.regtime

Subquery needs to go to another table, and SELECT address, city from t2 WHERE t2.mid = t1.mid AND MAX(t2.seqs)

t2 may contain multiple mids with different sequence numbers. So we want mids to match, and for seqs to be the highest.

Problem: returning multiple columns within 1 subquery while getting the highest t2.mid.

Desired end result:

 mid | regtime | address | city

Answer 1

with t as 
    ( select t2.mid, address, city 
        from t2 
       where t2.seqs = ( select max(tt.seqs) 
                           from t2 tt
                          where tt.mid = t2.mid ) 
     ) 
select t1.mid, t1.regtime, t.address, t.city from t1, t where t1.mid = t.mid

Should work.

HTH