Instrução única do MySQL para mesclar duas tabelas

select u.*, firstname, lastname
from user u join
     (select uf.user_id,
             max(case when key = 'firstname' then value end) as firstname,
             max(case when key = 'lastname' then value end) as lastname
      from user_field uf
      group by user_id
     ) uf
     on uf.user_id = u.id;

select u.*. firstname.value, lastname.value
from user u join
     user_field firstname
     on u.id = firstname.user_id and firstname.key = 'firstname' join
     user_field lastname
     on u.id = lastname.user_id and lastname.key = 'lastname';