如何保持安全并避免SQL注入？

Question

请查看我的SQL注入攻击代码。 请帮我避免SQL注入攻击。了解SQL注入。这段代码在SQL注入时是否安全？

<?php

$recno=$_GET['recno'];
$sql = 'set @parameter = (
    select group_concat(distinct
        concat(
            "sum(case when `parid`=\'", `parid`, "\' then `par_values` end) 
               as `", `parid`, "`"
        )
    )
    from #est_paaram_value
    where recno="'.$recno.'"
);
set @parameter = concat("select year as YEAR,qtr as QUARTER, ", @parameter, " 
from #est_paaram_value where recno=\''.$recno.'\' group by `year`,qtr");
prepare stmt from @parameter;
execute stmt;';
$result = $con->multi_query($sql);

if ($err=mysqli_error($con)) { echo $err."<br><hr>"; }
if ($result) {
  do {
  if ($par = $con->store_result()) {
      echo "<table id='myTable' name='myTable'>";
      echo "<thead><tr>";
       for($i=0; $i<mysqli_num_fields($par); $i++)
      {
          $field = mysqli_fetch_field($par);
          echo "<td bgcolor=lightgray><b>{$field->name}</b></td>";
      }
      echo "</tr>\n</thead>";
      echo "<tbody>";
      while($row = $par->fetch_row())
      {
          echo "<tr>";
          foreach($row as $par_val) {
            if ($par_val === NULL) { $par_val = '(null)'; }
            echo "<td>$par_val</td>";
          }
          echo "</tr>\n";
      }
      $par->free();
      echo"</tbody>";
      echo "</table>";
    }
  } while ($con->more_results() && $con->next_result());

}