MySQL Update Row from Query - mysql

Question

I've got a complex (For me!) query that basically is grabbing the data from 2 tables then grouping them togeher (This bit works 100% as a SELECT query)
But when I now need to update the database in another table, it won't work.
I've got this:
UPDATE
exp_channel_data data,
(
SELECT
posts.cat_id,
posts.entry_id,
cats.cat_name,
cats.cat_id,
GROUP_CONCAT('{"',cats.cat_name, '"}:{"',cats.cat_name,'"}') as category_tag
FROM
exp_category_posts posts,
exp_categories cats
WHERE
cats.cat_id = posts.cat_id
GROUP BY
posts.entry_id
) category
SET
data.field_id_178 = category.category_tag
WHERE
data.entry_id = category.entry_id;
But I'm getting this error:
Duplicate column name 'cat_id'
I think its because i'm trying to make the connection between the two tables, BUT it's not linking.
Like I said, the SELECT query works on it's own, but when put in to the UPDATE - It just throws this error.
:(

Answer 1

You are very close to what you need:
UPDATE exp_channel_data data JOIN
(SELECT posts.entry_id,
GROUP_CONCAT('{"',cats.cat_name, '"}:{"',cats.cat_name,'"}') as category_tag
FROM exp_category_posts posts JOIN
exp_categories cats
ON cats.cat_id = posts.cat_id
GROUP BY posts.entry_id
) category
ON data.entry_id = category.entry_id
SET data.field_id_178 = category.category_tag;
The important part was removing cats.cat_id from the subquery. You had two columns with that name, confusing MySQL.
I also fixed the query to use proper, explicit JOIN syntax.