Para este caso, você pode fazer assim;) Apenas um simples pivô de tabela.
select
users.id,
users.slug,
users.name,
max(if(users_info.info_key = 'email', users_info.info_val, null)) as email,
max(if(users_info.info_key = 'age', users_info.info_val, null)) as age
from users
inner join users_info
on users.id = users_info.parent
group by users.id
SQLFiddle DEMO AQUI
Se você tiver uma
info_key dinâmica , você precisará de um sql dinâmico para fazer isso, aqui dou um exemplo. SET @sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'max(if(users_info.info_key = ''',
users_info.info_key,
''', users_info.info_val, null)) as ',
users_info.info_key
)
) INTO @sql
FROM users
inner join users_info
on users.id = users_info.parent
;
SET @sql = CONCAT('select users.id, users.slug, users.name, ', @sql, ' FROM users
inner join users_info group by users.id having email = \'[email protected]\'');
PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
SQLFiddle DEMO AQUI
