SQL query to select posts from multiple categories

Question

I have built a query to select posts within a category. Works fine. But when I choose to add a secondary filter to exclude a category, the query return the same result set, as if the secondary category filter is being ignored.

In the following the query should select all posts in category 7 and exclude those in category 10:

$querystr = "SELECT * FROM $wpdb->posts 
LEFT JOIN $wpdb->term_relationships ON($wpdb->posts.ID = $wpdb->term_relationships.object_id)  
LEFT JOIN $wpdb->term_taxonomy ON($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)  

WHERE ($wpdb->term_taxonomy.term_id = 7
   AND $wpdb->term_taxonomy.term_id <> 10  
   AND $wpdb->term_taxonomy.taxonomy = 'category'   
   AND $wpdb->posts.post_type = 'post'  
   AND $wpdb->posts.post_status = 'publish')";

Can someone help?

Answer 1

I would use the built in API like Rarst mentioned. You could do something like this:

$just_seven = new WP_Query(
  array(
    'category__in' => array( 7 ),
    'category__not_in' => array( 10 )
  )
);

You would then have those items in $just_seven->posts.

However, if you MUST use a direct SQL statement, I'd suggest using INNER JOIN instead of LEFT JOIN.