Update a column with a COUNT of other fields is SQL?

Hi guys I have the following tables set up:

Articles:
ID | TITLE | CONTENT | USER | NUM_COMMENTS

COMMENTS
ID | ARTICLE_ID | TEXT

I need a sql statement which updates the NUM_Comments field of the articles table with teh count of the comments made against the article like:

update articles a, comments f 
set a.num_comments =  COUNT(f.`id`)
where f.article_id = a.id

The sql above doesn't work and I get an Invalid Use fo Group function error. I'm using MySQL Here.

---

You can't have a join in an update statement. It should be

update articles
set num_comments =
(select count (*) from comments
where comments.article_id = articles.id)

This will update the entire articles table, which may not be what you want. If you intend to update only one article then add a 'where' clause after the subquery.

---

This should work.

UPDATE articles a SET num_comments = 
(SELECT COUNT(*) FROM comments c WHERE c.article_id = a.id)

But i would rather update only one record when comment has been posted:

UPDATE articles a SET num_comments = 
(SELECT COUNT(*) FROM comments c WHERE c.article_id = 100) WHERE a.id = 100

---

To update based on a column count alone, you could do something like:

update articles, (select count (*) from comments where comments.article_id = articles.id) as newtotals set articles.num_comments = newtotals.count;

or ... if you had a situation that required rolling counts:

update articles, (select (count (*)) + (articles.num_comments) as count from comments join articles on comments.article_id = articles.id group by articles.id) as newtotals set articles.num_comments = newtotals.count;

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