Question

I am using the latest MySQL.

I have two tables: backlinks and pages.

I am trying to update a column called cache_links in the table pages for a certain url. The column should equal the number of rows in backlinks where destination = url.

I could do this in my application language like so, in case what I mean to do isn’t clear:

$myURL = "http://google.com/";

$numLinksResult = mysqli_query($c, "SELECT count(*) AS `num` FROM `backlinks` WHERE `same_domain` = FALSE AND `url` = '$myURL'");
$numLinksRow = mysqli_fetch_assoc($numLinksResult);
$numLinks = $numLinksRow['num'];

mysqli_query($c, "UPDATE `pages` SET `links` = $numLinks WHERE `url` = '$myURL'");

The query I’ve been trying to build currently looks like this:

UPDATE `pages`, `backlinks` SET pages.`cache_links` = backlinks.count(*)
    WHERE  backlinks.`destination` = pages.`url`
        AND backlinks.`same_domain` = FALSE
        AND pages.`url` = 'http://google.com/'

The problem, I think, is at backlinks.count(*). I have no idea what I’m doing wrong. The error I get is:

#1064 - You have an error in your SQL syntax; check the manual that corresponds
to your MySQL server version for the right syntax to use near '*)
WHERE backlinks.`destination` = pages.`url` AND backlinks.`same_' at line 1

Any help is greatly appreciated. Thanks a ton!

Answer 1

You can use a correlated sub query.

UPDATE `pages`
SET    pages.`cache_links` =
       (SELECT COUNT(*)
       FROM    backlinks
       WHERE   backlinks.`destination` = pages.`url`
       AND     backlinks.`same_domain` = FALSE
       )
WHERE  pages.`url` = 'http://google.com/'