Question

My case is as follow:

I have a table articles with these details:

    article | image1 | image2 | image3 
    ---------------------------------    
      1     | im-x
      2     | im-y
      3     | im-z 

and other article_images table with this data:

    article | image    
    ---------------    
        1   | im-a
        1   | im-b
        2   | im-c
        2   | im-d
        3   | im-e 

I need to update the table articles like this:

    article | image1 | image2 | image3    
    ---------------------------------    
       1    | im-x   | im-a   | im-b
       2    | im-y   | im-c   | im-d    
       3    | im-z   | im-e   | 

I know it seems not very difficult, but impossible to find an example on google. Can anyone help me, please?

Answer 1

This query will produce the desired Pivot:

SELECT a.article, coalesce(i1.image,'') AS img1, coalesce(i2.image, '') AS img2
  FROM (SELECT article FROM article_images GROUP BY article) a
  LEFT JOIN article_images i1 ON a.article = i1.article AND i1.image =
       (SELECT image FROM article_images WHERE article = a.article
         ORDER BY image LIMIT 1)
  LEFT JOIN article_images i2 ON a.article = i2.article AND i2.image =
       (SELECT image FROM article_images WHERE article = a.article
         ORDER BY image LIMIT 1 OFFSET 1);

A bit complicated, but this subquery will work in MySQL + PostgreSQL.

To update use (this UPDATE is MySQL specific):

UPDATE articles a, (SELECT a.article, coalesce(i1.image, '') AS img1,
                    coalesce(i2.image, '') AS img2
  FROM (SELECT article FROM article_images GROUP BY article) a
  LEFT JOIN article_images i1 ON a.article = i1.article AND i1.image =
       (SELECT image FROM article_images WHERE article = a.article
         ORDER BY image LIMIT 1)
  LEFT JOIN article_images i2 ON a.article = i2.article AND i2.image =
       (SELECT image FROM article_images WHERE article = a.article
         ORDER BY image LIMIT 1 OFFSET 1)) AS s
SET a.image2 = s.img1, a.image3 = s.img2
WHERE a.article = s.article;

If your database support window functions + CTE, like SQL Server, PostgreSQL or ORACLE, the following query can be used instead to generate Pivot:

WITH rowed AS (
  SELECT article, image,
         row_number() OVER (PARTITION BY article ORDER BY image) AS row
    FROM article_images)
SELECT a.article, coalesce(i1.image, '') AS img1, coalesce(i2.image, '') AS img2
  FROM (SELECT article FROM article_images GROUP BY article) AS a
  LEFT JOIN rowed i1 ON i1.article = a.article AND i1.row = 1
  LEFT JOIN rowed i2 ON i2.article = a.article AND i2.row = 2;

Now you have a single row per article in a shorter way and you can use this subquery to update:

UPDATE articles a
SET image2 = s.img1,
    image3 = s.img2
FROM (WITH rowed AS (
  SELECT article, image,
         row_number() OVER (PARTITION BY article ORDER BY image) AS row
    FROM article_images)
  SELECT a.article, coalesce(i1.image, '') AS img1, coalesce(i2.image, '') AS img2
     FROM (SELECT article FROM article_images GROUP BY article) AS a
      LEFT JOIN rowed i1 ON i1.article = a.article AND i1.ROW = 1
      LEFT JOIN rowed i2 ON i2.article = a.article AND i2.ROW = 2) AS s
WHERE a.article = s.article;

This UPDATE query will work in PostgreSQL, but likely it wont on ORACLE / SQL Server.

You can play around with MySQL and PostgreSQL variants.