u_id   profile_key   value

1      first_name    Jason
1      last_name     Smith
2      first_name    Sam
1      birth_date     01/01/1970
2      etc               etc

SELECT p.p_id, post, p.u_id, u.profile_key, u.value
FROM posts p
INNER JOIN users_profile u
on p.u_id = u.u_id

CREATE TABLE IF NOT EXISTS `users_profile` (
  `u_id` INT(10) UNSIGNED NOT NULL,
  `profile_key` VARCHAR(255) COLLATE utf8_unicode_ci NOT NULL,
  `profile_value` TEXT COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY  (`u_id`,`profile_key`)
) ENGINE=INNODB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
 
--
-- Contraintes pour les tables exportées
--
 
--
-- Contraintes pour la table `users_profile`
--
ALTER TABLE `users_profile`
  ADD CONSTRAINT `users_profile_ibfk_1` FOREIGN KEY (`u_id`) REFERENCES `users` (`u_id`);

SELECT u_id,
max(if(profile_key='first_name', VALUE, NULL)) as first_name,
max(if(profile_key='last_name', VALUE, NULL)) as last_name,
max(if(profile_key='birth_date', VALUE, NULL)) as birth_date,
max(if(profile_key='etc', VALUE, NULL)) as birth_date
FROM users_profile
GROUP BY u_id;