Custom SQL Query for Wordpress page

The wpdb object can be used to run arbitrary queries against the WordPress database. Let's say you want to list the most recent 4 posts:

$results = $wpdb->get_results( "SELECT * FROM $wpdb->posts WHERE `post_type`='post' LIMIT 4" );

The $wpdb->posts variable will output the table name for posts. It's usually wp_posts, but if you're using a custom database prefix this could be different.

However, if you're trying to grab post data (or custom post data or meta information or user data or ... anything built-in to WordPress), you should really be using WP_Query.

Here's the same query above, re-written for WP_Query:

$query = new WP_Query(
    array(
        'post_type'      => 'post',
        'posts_per_page' => 4
    )
);

The advantage of using WP_Query is that it will automatically join against the post meta table and return an array of WP_Post objects that you can work with and loop through.

<?php 
global $wpdb;
$post_id = $wpdb->get_results("SELECT post_id FROM $wpdb->postmeta WHERE (meta_key = 'mfn-post-link1' AND meta_value = '". $from ."')");
    print_r($post_id); /
?>

Define global variable $wpdb then make custom query and pass $wpdb->get_results();