select 
*
from table1 as a
left join table2 as b on a.UserID=b.UserID and not exists(select 1 from table2 where UserID=b.UserID and yearmonth>b.yearmont)

------解决方案--------------------

楼主有点点害人,在5楼把表名改了,列名也改了,数据也不写全.
SQL code
select m.userid , m.momeny , n.yearmonth , n.momeny
from test_table1 m left join test_table2 n
on m.userId = n.userId and n.yearmonth = (select max(yearmonth) from test_table2 t where t.userId = n.userId)

select m.userid , m.momeny , n.yearmonth , n.momeny
from test_table1 m left join test_table2 n
on m.userId = n.userId and not exists (select 1 from test_table2 t where t.userId = n.userId and t.yearmonth > n.yearmonth)

select m.userid , m.momeny , n.yearmonth , n.momeny
from test_table1 m left join
(
  select t.* , row_number() over(partition by userId order by yearmonth desc) px from test_table2 t
) n
on m.userid = n.userid and n.px = 1



create table test_table1(userid number(11) not null, momeny int null );

insert into test_table1 values(1,100);

insert into test_table1 values(2,200);

insert into test_table1 values(3,500);

create table test_table2(id number(11) not null,userid number(11) not null,yearmonth varchar2(6) not null, momeny int null );

insert into test_table2 values(1,1,201108,200);

insert into test_table2 values(2,1,201109,500);

insert into test_table2 values(3,2,201107,700);

insert into test_table2 values(4,2,201109,800);

insert into test_table2 values(5,2,201110,750);

select m.userid , m.momeny , n.yearmonth , n.momeny
from test_table1 m left join test_table2 n
on m.userId = n.userId and n.yearmonth = (select max(yearmonth) from test_table2 t where t.userId = n.userId)
/*
    USERID     MOMENY YEARMO     MOMENY
---------- ---------- ------ ----------
         1        100 201109        500
         2        200 201110        750
         3        500                  

3 rows selected.
*/

select m.userid , m.momeny , n.yearmonth , n.momeny
from test_table1 m left join test_table2 n
on m.userId = n.userId and not exists (select 1 from test_table2 t where t.userId = n.userId and t.yearmonth > n.yearmonth)
/*
    USERID     MOMENY YEARMO     MOMENY
---------- ---------- ------ ----------
         1        100 201109        500
         2        200 201110        750
         3        500                  

3 rows selected.
*/

select m.userid , m.momeny , n.yearmonth , n.momeny
from test_table1 m left join
(
  select t.* , row_number() over(partition by userId order by yearmonth desc) px from test_table2 t
) n
on m.userid = n.userid and n.px = 1

/*
    USERID     MOMENY YEARMO     MOMENY
---------- ---------- ------ ----------
         1        100 201109        500
         2        200 201110        750
         3        500                  

3 rows selected.
*/