使用“搜索”按钮过滤数据库

我这里有我的代码，并且我有这个错误：

警告：mysql_query（） 期望参数 1 为字符串，资源在 C：''xampp''htdocs''xampp''elev8log''pages''tables''task.php 第 396 行给出

这是我的代码：

 <?php
            
                    $sql = mysql_query('select * from taskview where dept_id=(select deptid from users where username="'.$_SESSION['username'].'")');
                      //where empname="'.$_SESSION['username'].'"  and req_dateapprove IS NULL');
//where req_approveuser="'.$_SESSION['username'].'" and req_dateapprove IS NULL'
            if(isset($_POST['search'])) {
              $search_term = mysql_real_escape_string($_POST['search_box']);
              $search_term=mysql_real_escape_string($_POST['search_box']);
              $sql.="WHERE req_code LIKE '%{$search_term} %'";
              $sql.="OR req_date LIKE '%{$search_term} %'";
              $sql.="OR req_status LIKE '%{$search_term} %'";
              $sql.="OR req_dateneeded LIKE '%{$search_term} %'";
              $sql.="OR req_approveuser LIKE '%{$search_term} %'";
               $sql.="OR req_noteuser LIKE '%{$search_term} %'";
                $sql.="OR req_datenote LIKE '%{$search_term} %'";
                 $sql.="OR form_name LIKE '%{$search_term} %'";
                  $sql.="OR itrf_type LIKE '%{$search_term} %'";
                   $sql.="OR username LIKE '%{$search_term} %'";
                    $sql.="OR itrf_details LIKE '%{$search_term} %'";
                     $sql.="OR itrf_date LIKE '%{$search_term} %'";
                      $sql.="OR comp_name LIKE '%{$search_term} %'";
                       $sql.="OR dept_name LIKE '%{$search_term} %'";
                        $sql.="OR dept_code LIKE '%{$search_term} %'";
                         $sql.="OR dept_id LIKE '%{$search_term} %'";
             // $sql.="OR descriptionsql LIKE '%{$search_term}%'";  
} 
 //THIS IS THE LINE 396------->>>>>>$query = mysql_query($sql) or die(mysql_error());
          
          
          ?>
        <!-- Main content -->
        <section class="content">
          <div class="row">
            <div class="col-xs-12">
              <div class="box">
                <div class="box-header">
                  <h3 class="box-title">List of Pending Task</h3>
                </div><!-- /.box-header -->
                <div class="box-body">
                  <table id="example2" class="table table-bordered table-hover">
                            <label><input type="radio" name="all" value="all" checked>&nbsp;All&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</label> 
                            <label><input type="radio" name="all" value="wait">&nbsp;Waiting for Feedback&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</label><thead>
                            <label><input type="radio" name="all" value="closed">&nbsp;Closed</label><thead>
                            <form name="search_form" method="POST" action="task.php">
                            Search: <input type="text" name="search_box" value="" />
                            <input type="submit" name="search" value="search....">
                          </form>
                      <tr>
                        
                        <th>Date of Request</th>
                        <th>Requestor Name</th>
                        <th>Form Type</th>
                        <th>Request Type</th>
                        
                         <th>Date Needed</th>
                          <th>Status</th>
                        <th>Action</th>
                         
                      </tr>
                    </thead>
             
                <?php
                     // if(isset($_GET['req_code'])){
                   //   }
while($row = mysql_fetch_array($query))
{
?>
<tr>
      
       <td><?php echo date($row['req_date']); ?></td>
      <td class="left"><?php echo htmlentities($row['username'], ENT_QUOTES, 'UTF-8'); ?></a></td>
      <td><?php echo htmlentities($row['form_name'], ENT_QUOTES, 'UTF-8'); ?></td>
      <td> <?php echo ($row['itrf_type']);?></td>
      
      <td> <?php echo ($row['req_dateneeded']);?></td>
      <td> <?php echo ($row['req_status']);?></td>
       
       <td class="left"><a href="#proceed-<?php echo $row['req_code'];?>" data-toggle="modal"><button type="submit" class="btn btn-primary" name="approved">Manage Task</button></a>
     
      
   </tr>