如何防止使用php在mysql中插入两次相同的记录

Question

The following is my code that I have written for not inserting same data I would like if the record exist in mysql then it should show me error message that the record already exist the else part should insert record to database but it not working can any one help me plz the help would be highly appreciated

function addcontact()
    {
        if(isset($_POST['addContact']))
        {
            $officeName = strip_tags($_POST['office_name']);
            $contactName = strip_tags($_POST['contactName']);
            $contactNo = strip_tags($_POST['contactNo']);
            $digitalNo = strip_tags($_POST['digitalNo']);
        $mobileNo = strip_tags($_POST['mobileNo']);

        $check="SELECT * FROM contacts WHERE office_name = '$officeName'";
        if(mysql_num_rows($check) != 0)
         {
            echo "Already in Exists<br/>";
        }else
                     {          
                        $sql = mysql_query("INSERT INTO contacts (office_name, contact_no, 
                          digital_no, mobile_no) VALUES 
                         ('$contactName','$contactNo','$digitalNo','$mobileNo')") or die(mysql_error());
        if($sql)
        {
            header("Location: index.php?admin&done");  exit;
        }
        else
        {
            header("Location: index.php?admin&failed");  exit;  
        }
    }
}
}

Answer 1

you did mistake here.

$check="SELECT * FROM contacts WHERE office_name = '$officeName'";
    if(mysql_num_rows($check) != 0)
     {
        echo "Already in Exists<br/>";
    }

just add mysql_query like

$check=mysql_query("SELECT * FROM contacts WHERE office_name = '$officeName'");
    if(mysql_num_rows($check) != 0)
     {
        echo "Already in Exists<br/>";
    }

or you can also use like

    $name=$_POST['username'];

$q="select * from login where name='$name' ";
$rs=mysql_query($q);
if(mysql_fetch_row($rs)>0)
{
    echo "already exist";
}
else
{
    $msg="done";
}