当插入查询同时多次调用时，避免多次插入

$Link= mysql_connect("localhost", "root", "", "");
query = mysql_query($Link,"SELECT col_name FROM table_name WHERE col_name='$val'"); 
$num=mysql_num_rows($query);
if($num==0){   
    $query1 = mysql_query($Link,"INSERT INTO table_name(col_name)VALUES('$val')"); )
}
$Link= mysql_connect("localhost", "root", "", "