SQL查询值从$ _GET更改

如何从Php修改MySQL查询，因此，如果选择ALL，将不会出现Where语句，如果选择某些用户登录，将仅显示该用户记录？我需要，例如，如果我选择“<option value="All">ALL</option>查询将不带WHERE语句”，但是例如，如果我选择了“<option value="admin">Admin</option>将存在此查询”Where tsales_funnel.Darijuma_vaditajs = 'admin'

SQL语句：

$sql = "Select
  tsales_funnel.ID As ID,
  wp_users.display_name As Darijuma_vaditajs,
  tcportal_starpnieks.Cp_Name As Starpnieks,
  tcportal_stucture.Cp_Name As OWCA,
  n_products.Product_Nos As Produkts,
  tsales_funnel_mrecord.Deadline As Deadline,
  tsales_funnel_clients.Reg_nr As Klienta_Regnr,
  tfirmas_reg.name_in_quotes As Klients,
  tsales_funnel_mrecord.ID As Prod_ID,
  tsales_funnel.Statuss As Statuss
From
  tsales_funnel Left Join
  tsales_funnel_mrecord On tsales_funnel_mrecord.Funnel_ID = tsales_funnel.ID
  Left Join
  tcportal_starpnieks On tcportal_starpnieks.Cp_code = tsales_funnel.Starpnieks
  Left Join
  tcportal_stucture On tcportal_stucture.Cp_code = tsales_funnel.OWCA Left Join
  tsales_funnel_clients On tsales_funnel_clients.Funnel_ID = tsales_funnel.ID
  Left Join
  tfirmas_reg On tfirmas_reg.regcode = tsales_funnel_clients.Reg_nr Left Join
  n_products On tsales_funnel_mrecord.Product_type = n_products.Product_Code
  Left Join
  wp_users On tsales_funnel.Darijuma_vaditajs = wp_users.user_login ".$sqlselecteduser."
Order By
  Deadline";

表单中的GET方法：

<?php
if(isset($_GET['selecteduser'])){
    $selecteduser = esc_sql($_GET['selecteduser']);
} else {
    $selecteduser = $_SESSION['selecteduser'];
}
if($selecteduser == 'all') {
    $sqlselecteduser = '';
} else {
    $sqlselecteduser = " Where tsales_funnel.Darijuma_vaditajs = $selecteduser ";
}

?>

发送请求此表的表格：

<form action="" method="GET" id="selectdo">
<table border="1">
    <tr>
        <td>
            <label for="selector1">Darījuma vadītājs:</label>
        </td>
        <td>
            <select id="selector1" name="selecteduser">
            <option value="all">-ALL-</option>
            <?php 
                $sql = "SELECT display_name, user_login FROM wp_users";
                $results = $wpdb->get_results($sql);  // return an object, not ARRAY_N
            if ($results) {
                foreach ($results as $row) {
                    echo "<option value = '".$row->user_login."'>".$row->display_name."</option>"; 
            }}
                echo "</select></br>";                          
            ?>
        </td>
        <td><input type="submit" value="ok" /></td>
    </tr>
</table>
</form>

之后，数据显示到表格中。