Auto set Post to Specific Categories

Question

Here is some rough code for you, you may improve the query if you like…

By the way selecting post_title and post_status is not neccessary, but useful if you want to ensure that you are returning the right results via conditional logic within your foreach loop.

global $wpdb;

$results = $wpdb->get_results(
    "
    SELECT ID, post_title, post_status
    FROM {$wpdb->prefix}posts
    WHERE post_status="publish"
    AND (post_title LIKE 'mobile'
    OR post_title LIKE 'Nokia'
    OR post_title LIKE 'Samsung')
    "
);

foreach($results as $result) {
    wp_set_object_terms($result->ID, array('Mobile'), 'my_taxonomy');
}

Update

Based on your comment:

The problem is I need to do this for nearly 100 categories. I can write 100 queries for it. The problem is, will I have to face a performance problem?

You don’t have to write 100 queries for this, instead you can abstract the process into something more manageable.

If you know the categories and the titles that you want to associate with the categories then you can create an array like this following by logic to iterate over your categories which forms the SQL statement, executes the query and then updates the post objects based on the results:

set_time_limit(0); //hopefully by pass your server config

global $wpdb;

$categories = array(

    //category          post titles to search for
    //  ↓                 ↓        ↓         ↓
    'mobile' => array('Nokia', 'Samsung', 'Mobile'),
    'tablets' => array('iPad', 'iPad mini', 'Surface Pro 4')

    //etc...

);

$sql =<<<DOC
    SELECT ID, post_title, post_status
    FROM {$wpdb->prefix}posts
    WHERE post_status="publish"
    AND (%s)
DOC;

foreach($categories as $category => $titles) {

    $like = array();

    foreach($titles as $title) {
        $like[] = "post_title LIKE '{$title}'";
    }

    $results = $wpdb->get_results( sprintf($sql, implode(' OR ', $like)) );

    foreach($results as $result) {
        wp_set_object_terms($result->ID, array($category), 'my_taxonomy');
    }

}

If you have to run many queries, a few hundred or thousands even, then you should consider batch processing/cron job execution.

If you want case-insensitive queries then you can do:

$like[] = "LOWER(post_title) LIKE LOWER('{$title}')";

Recommended reading:

Update