mysql，在另一列中选择两个具有相同id但值不同的不同行

Question

Preemptive apology for the title there... wasn't sure what to put....

I am not the best with mysql and I have a problem that I am not able to wrap my head around.

I have one table....which is the metadata table for a wordpress site... In the table I am storing the FB like count and the permalink for each post.... so it looks something like this for one post...

+-------------+----------------+-----------------+----------------------------------+
| meta_id     |    post_id     |     meta_key    |            meta_value            |
+-------------+----------------+-----------------+----------------------------------+
|           1 |      446       |    _fb_count    |                 2                |
|           2 |      446       |    _permalink   | /2013/08/image-aligned-left/446/ |
+-------------+----------------+-----------------+----------------------------------+

What I would like to do is query this table and return both these values so that the returned data would be (post_id,Facebook Like Count, Permalink)... So it would look like:

+----------------+-----------------+----------------------------------+
|    post_id     |     _fb_count   |            _permalink            |
+----------------+-----------------+----------------------------------+
|      446       |        2        | /2013/08/image-aligned-left/446/ |
+----------------+-----------------+----------------------------------+

I looked around and am guessing I need to do some kind of inner join on the same table.... but I am not getting the results I want...

I tried something like this with no success:

SELECT pl.post_id,pl.meta_value,FBL.meta_value from wp_postmeta pl inner join wp_postmeta FBL on pl.post_id = FBL.post_id WHERE pl.meta_key = '_permalink' OR FBL.meta_key = '_fb_count

Any help would be appreciated...

Answer 1

You have to use conditional aggregation for that

SELECT post_id,
       MAX(CASE WHEN meta_key = '_fb_count' THEN meta_value END) `_fb_count`,
       MAX(CASE WHEN meta_key = '_permalink' THEN meta_value END) `_permalink`
  FROM wp_postmeta
 WHERE meta_key IN('_fb_count', '_permalink')
 GROUP BY post_id

Output:

| POST_ID | _FB_COUNT |                       _PERMALINK |
|---------|-----------|----------------------------------|
|     446 |         2 | /2013/08/image-aligned-left/446/ |

Here is SQLFiddle demo

But I agree with @Rob you probably better off using WP's appropriate functions for that.

Answer 2

Use following query:

SELECT e.post_id, e.meta_value as _fb_count, m.meta_value as _permalink FROM table e inner join 'table' m where e.meta_key = '_fb_count' and m.meta_key= '_permalink' and e.post_id= m.post_id