SQL Converting table -> changing repeating attributes into columns

Question

I'm creating a site using WooCommerce and I would like to integrate it with my warehouse.

I need to convert a table that looks like that:

   ----------------------------
id |post_id| meta_key|meta_value |
   ----------------------------
 1 | 1     |_stock   |1          |
 2 | 1     |_price   |10         |
 3 | 1     |X        |X          |
 4 | 2     |_stock   |2          |
 5 | 2     |_price   |8          |
 6 | 2     |X        |X          |

into:

   -------------------------
   |post id|_stock | _price  |
   -------------------------
   | 1     |1      |10       |
   | 2     |2      |8        |

I tried:

SELECT post_id, meta_value AS stock
FROM wp_postmeta
JOIN
(
SELECT post_id, meta_value AS regular_price
FROM wp_postmeta
WHERE wp_postmeta.meta_key = '_regular_price' 
)
WHERE wp_postmeta.meta_key = '_stock'
ORDER BY post_id ASC

and got:

'#1248 - Every derived table must have its own alias

Is there any way that avoids create table ?

I've just started my adventure with sql and sorry if the answer is obvious. And sorry if the title is misleading.

Thank You in advance.

Answer 1

Just use conditional aggregation:

select post_id,
       max(case when meta_key = '_stock' then meta_value end) as stock,
       max(case when meta_key = '_regular_price' then meta_value end) as regular_price
from wp_postmeta
where meta_key in ('_stock', '_regular_price')
group by post_id;