PHP/MySqlのクエリおよび/または高度な検索

どのフィールドが設定されているかを確認し、実行時にそれに応じてクエリを実行できます。このようなことができます：

$query = "";
$keyword = $_REQUEST['keyword'];
$country = $_REQUEST['country'];
$category = $_REQUEST['category'];
if(isset($keyword)){//if keyword set goes here
   $query = "SELECT * FROM table1 WHERE Name LIKE '%$keyword%' OR ZipCode LIKE '%$keyword%' OR country LIKE '%$keyword%'";
   if(isset($category)){
     $query .= "AND category LIKE '$category'";
   }
   if(isset($country)){
     $query . = "AND country LIKE '$country'"
   }
}else if (isset($category)){ //if keyword not set but category set then goes here
  $query = "SELECT * FROM table1 WHERE category LIKE '$category'";
  if(isset($country)){
    $query . = "AND country LIKE '$country'";
  }
}else if(isset($country)){//if only country set goes here
  $query = "SELECT * FROM table1 WHERE country LIKE '$country'"
}