How to retrieve elements from another table in a SQL query?

With a WP_Query() you can have all of them what you want in an easy way. See all the arguments available in the Codex. With the following code, we are grabbing all(-1) the posts from wp_posts table, and using the post ID we are grabbing the postmeta items.

<?php
$productquery = new WP_Query( array( 'post_type'=>'wpcproduct','post_status'=>'publish','posts_per_page'=>-1 ) );

if( $productquery->have_posts() ) :
   while( $productquery->have_posts() ) : $productquery->the_post();
      global $post;
      $post_id = $post->ID;

      echo '<h2>'. $post_id .' - '. get_the_title() .'</h2>';
      echo '<img src="'. get_post_meta( $post_id, 'product_img1', TRUE ) . '" alt="Product Image 1">';
      echo '<img src="'. get_post_meta( $post_id, 'product_img2', TRUE ) . '" alt="Product Image 2">';
      echo '<img src="'. get_post_meta( $post_id, 'product_img3', TRUE ) . '" alt="Product Image 3">';

   endwhile;
else :
   echo __( 'No Product\'s found', 'textdomain' );
endif;
wp_reset_postdata(); //reset the query after use

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