SELECT t1.meta_id, 
           t1.post_id, 
           t1.meta_key, 
           t1.meta_value, 
           t2.meta_key, 
           t2.meta_value 
    FROM table t1, table t2  
    WHERE t1.post_id = t2.post_id 
    AND t1.meta_key = '_apais'
    AND t2.meta_key = '_aciudad'
    ORDER BY t1.meta_key

我不确定这是否是您要查找的内容，或者JOIN ON字段是否正确，因为您的问题中它不是很清楚，但是这个查询怎么样？

SELECT City.meta_id
     , City.post_id
     , City.meta_key
     , City.meta_value
     , Country.meta_key
     , Country.meta_value
  FROM yourTable City
INNER JOIN yourTable Country ON City.post_id = Country.post_id
WHERE City.meta_key = '_aciudad' AND Country.meta_key = '_apais'
ORDER BY Country.meta_value, City.meta_value

SELECT  mc.*, mp.meta_key, mp.meta_value
FROM    meta mc
JOIN    meta mp
ON      mp.post_id = mc.post_id
        AND mp.meta_key = '_apais'
WHERE   mc.meta_key = '_aciudad'
ORDER BY
        mp.meta_value, mc.meta_value

尝试使用自联接。要使其工作，您必须为表使用别名，以便可以引用它两次。
大概是这样的

select

  t1.meta_id, t1.post_id,
  t1.meta_key, t1.meta_value, 
  t2.meta_key, t2.meta_value

from TableName t1
  inner join TableName t2
  on t1.meta_id = t2.meta_id and t1.post_id = t2.post_id
    and t1.meta_key = "_aciuidad" and t2.meta_key = "_apais"