delete all but top

create table blogs
(id integer, 
 user_id integer);

insert into blogs values
(1,1),
(2,2),
(3,3);

create table entries
(id integer, 
 blog_id integer, 
 title varchar(32), 
 created_at timestamp);

insert into entries values
(1,1,'bobs first blog','2009-01-01'),
(2,1,'bobs second blog','2009-02-01'),
(3,1,'bobs third blog','2009-03-01'),
(4,2,'susans first blog','2009-04-01'),
(5,2,'susans second blog','2009-04-02'),
(6,2,'susans third blog','2009-04-03');