MySQL question: Appending text to a wordpress post, but only if it's in a certain category

I need to amend (via CONCAT, presumably) something to every wordpress post if it belongs to a certain category (say, category ID 7), but I'm struggling to get it to work.

To test, I'm first tr开发者_C百科ying to select all the relevant posts. So far, I have the following:

SELECT post_title 
    FROM cruise_wp_posts 
        LEFT JOIN cruise_wp_term_relationships 
        ON  cruise_wp_term_relationships.object_id = cruise_wp_posts.ID
        WHERE term_taxonomy_id = 87;

However, it only lists posts that are only in category 87 - I need all posts that are in category 87 (and possibly other categories too)

I'm a MySQL newbie, and this is really breaking my brain.

Any pointers would be passionately welcomed.

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The best way to do it is to filter it in as needed. This way the addition is made everywhere the_content is used and not just in the templates you modify.

<?php

function my_content_concat($the_content) {
    if (in_category(7)) {
        $the_content .= '<br /><br />foo!';
    }
    return $the_content;
}
add_filter('the_content', 'my_content_concat', 9);

?>

in_category can take the id, name or slug of your target category.

I put the filter at 9 so that it runs before WordPress texturizes the content. If you don't need that run it at 11.

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Why not just use get_the_category( $id ) and ammend the text when you output the post?

$cat = get_the_category( $postID );
if ($cat == 7) {
  //Add text here
}