也许：
使用左连接一次以获取meta3值（如果存在），确保所有包含meta1和meta2的文章都具有所需的值。然后使用exist和having来确保只获得同时具有meta1和meta2的记录，并且这些记录都具有所需的值。
未经测试。。。

SELECT P.ID, P.Title, PM.Meta_value
FROM Posts P
LEFT JOIN Post_MetaData PM
 on P.ID = PM.Post_ID
and PM.key = 'meta_3'
WHERE exists (SELECT 1 
              FROM post_meta 
              WHERE ((Key 'meta_1' and meta_value = 'Value_1') OR
                    (Key 'meta_2' and meta_value = 'Value_2'))
                and P.ID = Post_ID  --Either here or an AND in the HAVING clause...need to test to know

              GROUP BY  Post_ID
              HAVING count(*) = 2 )
ORDER BY -PM.meta_value desc, P.ID

这确实假设post\u元数据对每个post\u id的键有唯一的约束，否则我们可以得到值为a的meta\u 1和值为a的meta1，计数（*）将是2；并在结果中错误地返回它。
为确保空值是最后一个，请遵循此方法；mysql orderby一个数字，最后为空
做这件事是为了。。。。但我想会慢一点。

SELECT P.ID, P.Title, PM.Meta_value
FROM Posts P
LEFT JOIN Post_MetaData PM
 on P.ID = PM.Post_ID
and PM.key = 'meta_3'
WHERE P.ID in (SELECT Post_meta.Post_ID
              FROM post_meta 
              WHERE ((Key 'meta_1' and meta_value = 'Value_1') OR
                    (Key 'meta_2' and meta_value = 'Value_2'))
              GROUP BY  Post_ID
              HAVING count(*) = 2 )
ORDER BY -PM.meta_value desc, P.ID