index.php
<?php
global $wpdb;
$args = array( 'posts_per_page' => -1, 'post_limits' => -1, 'post_type' => 'post' );
$wp_query = new WP_Query( $args );
?>
<?php if( $wp_query->have_posts()) : ?>
<? while($wp_query->have_posts()) : $wp_query -> the_post(); ?>
<?php
$str_result = "SELECT post_title FROM $wpdb->posts WHERE ID = ". $use_post->ID;
$var = $wpdb->get_var( $str_result );
$row = $wpdb->get_row( $str_result );
print_r( $var );
print_r( $row );
?>
<?php endwhile; ?>
<?php endif; ?>
グローバル変数（クラス？）の $wpdb を使うっぽい。