create table city
(
idcity integer not null primary key,
cityname varchar( 50 )
);

create table population
(
idcity integer references city(idcity),
population bigint,
updatedate date
);


insert into city values ( 1 ,'город 1'),( 2 ,'город 2');

insert into population values 
( 1 , 500000 ,'2007-01-01'),( 1 , 600000 ,'2008-01-01'),( 2 , 300000 ,'2007-01-01'),( 2 , 200000 ,'2008-01-01'),( 1 , 500000 ,'2008-02-01');

select c.idcity, c.cityname,
       p.population, p.updatedate from city as c,
                                       population as p
where c.idcity = p.idcity and 
      p.updatedate = (select max(updatedate) from population as pp where pp.idcity = p.idcity)
order by c.idcity;
---------------------------------------------------------------------------------------------
IDCITY      CITYNAME                                           POPULATION           UPDATEDATE
----------- -------------------------------------------------- -------------------- ----------
           1  город  1                                                            500000   01 . 02 . 2008 
           2  город  2                                                            200000   01 . 01 . 2008