"Unknown column" error in a join query designed to select a single row within groups of interest

Question

May I know why I am getting error

Unknown column 'wp_postmeta.post_ID' in 'field list'

Here are the tables in question:

create table wp_posts (
  ID integer primary key auto_increment,
  post_title varchar(30),
  post_type varchar(30)
);

create table wp_postmeta (
  ID integer primary key auto_increment,
  post_id integer,
  meta_key varchar(30) not null default '_regular_price',
  meta_value integer not null
);

insert into wp_posts (post_title, post_type) values
('Apple Pie','Product'),
('French Toast','Product'),
('Shepards Pie','Product'),
('Jam Pie','Product'),
('Jam Pie','Product'),
('Plate','Not a Product'),
('Bucket','Not a Product'),
('Chequebook','Not a Product'),
('French Toast','Product'),
('French Toast','Product');

insert into wp_postmeta (post_id, meta_value) values
(1,10),
(2,5),
(3,9),
(4,8),
(5,11),
(6,12),
(7,10),
(8,6),
(9,1),
(10,1);

Here is the query:

SELECT wp_postmeta.post_ID, wp_posts.post_title, wp_postmeta.meta_value FROM (
SELECT wp_posts.post_title, MIN(wp_postmeta.meta_value) AS minprice
FROM wp_postmeta 
  JOIN wp_posts ON wp_postmeta.post_ID=wp_posts.ID
  WHERE wp_posts.post_type = 'Product'
GROUP BY wp_posts.post_title
)
as x inner join 

(
SELECT wp_postmeta.post_id, wp_posts.post_title, wp_postmeta.meta_value
FROM wp_postmeta 
  JOIN wp_posts ON wp_postmeta.post_ID=wp_posts.ID
  WHERE wp_posts.post_type = 'Product'
  ORDER BY wp_posts.post_title, wp_postmeta.meta_value
)
 as f on x.post_title = f.post_title and f.meta_value = x.minprice

fiddle with data

In case you're interested, I'm adapting this code from this tutorial that will give you the context of what I am doing.

Answer 1

you have to use subquery alias name in selection as you used subquery

SELECT f.post_ID, x.post_title, f.meta_value
FROM (
SELECT wp_posts.post_title, MIN(wp_postmeta.meta_value) AS minprice
FROM wp_postmeta 
  JOIN wp_posts ON wp_postmeta.post_ID=wp_posts.ID
  WHERE wp_posts.post_type = 'Product'
GROUP BY wp_posts.post_title
)
as x inner join 

(
SELECT wp_postmeta.post_id, wp_posts.post_title, wp_postmeta.meta_value
FROM wp_postmeta 
  JOIN wp_posts ON wp_postmeta.post_ID=wp_posts.ID
  WHERE wp_posts.post_type = 'Product'
  ORDER BY wp_posts.post_title, wp_postmeta.meta_value
)
 as f on x.post_title = f.post_title and f.meta_value = x.minprice

fiddle demo