title: Update wp_postmeta meta_value of certain meta_key with meta_value of other meta_key if post_ids are the same
body: <p>I have searched but I can't make this to work by myself</p>
  
  <p>So to be completely clear I have the WP database where I have meta_keys like these :</p>
  
  <pre><code>ID ------ POST_ID -------- META_KEY ---- META_VALUE
  1  ------   235  -------  book_price ---- 35$
  2  ------   256  -------  book_price ---- 65$
  3  ------   235  -------  tcp_book_price ---- 0
  4  ------   256  -------  tcp_book_price ---- 0
  </code></pre>
  
  <p>I want the meta_value of tcp_book_price to receive the meta_value from book_price with the same post_id</p>
  
  <p>End Result wished :</p>
  
  <pre><code>ID ------ POST_ID -------- META_KEY ---- META_VALUE
  1  ------   235  -------  book_price ---- 35$
  2  ------   256  -------  book_price ---- 65$
  3  ------   235  -------  tcp_book_price ---- 35$
  4  ------   256  -------  tcp_book_price ---- 65$
  </code></pre>
  
  <p>I have tried UPDATE FROM SELECT but I can't make it to work..</p>
  
  <p><strong>Thanks in advance</strong></p>
  
  <p>Ok I found the solution by myself with a PHP Script :</p>
  
  <pre><code>&lt;?php
  
  $conn = mysql_connect("localhost", "root", "root");
  
  if (!$conn) {
  echo "Impossible de se connecter à la base de données : " . mysql_error();
     exit;
  }
  
  if (!mysql_select_db("database_name")) {
     echo "Impossible de sélectionner la base database_name : " . mysql_error();
     exit;
  }
  
  $sql = "SELECT `post_id`, `meta_value` FROM `wp_postmeta` WHERE `meta_key` = 'book_price'";
  
  // query the database
  $resource = mysql_query($sql);
  
  // loop through the results
  while ($result = mysql_fetch_assoc($resource)) {
      // grab the value
      $price = $result['meta_value'];   // you should use quotes here, btw
      $pid = $result['post_id'];   // you should use quotes here, btw
  
      echo $price;
      echo $pid;
  
      // make your modifications;
  //    ++$nox; // or whatever
  
      // build the query
      $sql = "UPDATE `wp_postmeta` SET `meta_value` = '$price' WHERE `meta_key` = 'tcp_price' AND `post_id` = '$pid'";
  
  
      // run the query
      mysql_query($sql);
  }
  
  ?&gt;
  </code></pre>