您只需添加DISTINCT 到您的SQL查询，类似于：

$cities = $wpdb->get_col("SELECT DISTINCT(meta_value) FROM $wpdb->usermeta WHERE meta_key = \'my_cities_meta_key\'" );

或者，如果出于某种原因希望使用php执行此操作（如果希望知道一个城市在只显示唯一条目之前会列出两次）

$cities = $wpdb->get_col("SELECT meta_value FROM $wpdb->usermeta WHERE meta_key = \'my_cities_meta_key\'" );
$cities = array_keys(array_flip($cities));

如果您只想使用SQL根据用户角色进行筛选，类似的方法应该可以实现，但有点不太成熟，因为WP将用户角色存储在序列化数组中：

$cities = $wpdb->get_col("SELECT Distinct(c.meta_value) 
    FROM $wpdb->usermeta AS r 
    INNER JOIN $wpdb->usermeta AS c
      ON r.user_id = c.user_id
    WHERE r.meta_value LIKE \'%subscriber%\'
    AND c.meta_key = \'my_cities_meta_key\'");

要添加基于多个元键的过滤器，需要执行更多操作INNER JOIN, e、 g.对于您的区域元键：

$cities = $wpdb->get_col("SELECT Distinct(c.meta_value) 
    FROM $wpdb->usermeta AS r 
    INNER JOIN $wpdb->usermeta AS c
      ON r.user_id = c.user_id
    INNER JOIN $wpdb->usermeta AS re
      ON r.user_id = re.user_id
    WHERE r.meta_value LIKE \'%subscriber%\'
    AND c.meta_key = \'my_cities_meta_key\'
    AND re.meta_key = \'region\'");