MySQL find Array Values in DB Array field

Because your data is stored as a PHP serialized array you'll need to either pull the generic data out first and loop over that, or you'll need to perform a LIKE query. This post goes into things deeper but I'll show you how to do the LIKE query for WordPress.

WordPress has a dedicated function for properly escaping LIKE parameters, called wpdb::esc_like. After running through this, the SQL should look something like this:

SELECT
    user_id
FROM
    wp_usermeta
WHERE
    meta_key='addition_qualification'
    AND
    (
        meta_value  LIKE %s
        OR
        meta_value  LIKE %s 
    )

Once you call prepare on that SQL it will get transformed into:

SELECT
    user_id
FROM
    wp_usermeta
WHERE
    meta_key='addition_qualification'
    AND
    (
        meta_value  LIKE '%Allgemeine Pharmazie%'
        OR
        meta_value  LIKE '%Geriatrische Pharmazie%'
    )

This isn't the most ideal solution but it is probably the best for working with this type of data in a WordPress context. Here's the code that creates the above:

$zuqual = $this->userInput["Zuquali"];

if (!empty($zuqual)) {
    $likeTemplate = ' meta_value  LIKE %s ';
    
    // This will hold the above string repeated once for each item in our search array
    $likes = [];
    
    // This will hold sanitized values to perform LIKE searches, each surrounded by percent signs
    $params = [];
    foreach ($zuqual as $item) {
        $likes[] = $likeTemplate;
        $params[] = '%' . $this->wpdb->esc_like($item) . '%';
    }
    
    // If we have more than one search term, this will join with the OR, otherwise it will be left as-is
    $likeSql = implode(' OR ', $likes);
    
    // Create our query, remembering to surround the nested part with parentheses
    $sql = "SELECT user_id FROM wp_usermeta WHERE meta_key='addition_qualification' AND (" . $likeSql . ")";
    
    // Pass our escaped params in
    $prepared = $this->wpdb->prepare($sql, $params);
    $result = $this->wpdb->get_col($prepared);
}