Undefined index error in php (dropdown filters do not work)

Hey I searched a lot about this problem but everything I tried did not work. :(

My code has 3 dropdowns and they should work like search-filters but every time I select an option from the dropdown i get the undified index error for 3 lines & if I change another dropdown the other two filters get ignored... :(

This only the code for the first dropdown:

<?php  
  //when the filter changes, this php is called

 $output = '';


if(isset($_POST["businessUnit"]))  
{  
    if($_POST["businessUnit"] != '')  
    {
        if($_POST["productGroup"] != '') //first undefined index
        {
            $sql = "SELECT * FROM item WHERE businessUnit =         '".$_POST["businessUnit"]."' and productGroup = '".$_POST["productGroup"]."'";
        }
        else if($_POST["deviceType"] != '') //second undefined index
        {
            $sql = "SELECT * FROM item WHERE businessUnit = 
'".$_POST["businessUnit"]."' and productGroup = '".$_POST["deviceType"]."'";
        }
        else if($_POST["productGroup"] != '' && $_POST["deviceType"] != '') //third undefined index error
        {
            $sql = "SELECT * FROM item WHERE businessUnit = '".$_POST["businessUnit"]."' and productGroup = '".$_POST["productGroup"]."' and deviceType = '".$_POST["deviceType"]."'";
        }
        else
        {
            $sql = "SELECT * FROM item WHERE businessUnit = '".$_POST["businessUnit"]."'";
        }  
    }
    else  
    {  
        $sql = "SELECT * FROM item";  
    }  
    $result = sqlsrv_query($connect, $sql);  
    while($row = sqlsrv_fetch_array($result))  
    {  
        $output .= "<tr><td>".
                $row['businessUnit']."</td><td>".
                $row['productGroup']."</td><td>".
                $row['deviceType']."</td><td>".
                $row['serialNumber']."</td><td>".    
                $row['location']."</td><td>".
                $row['condition']."</td><td>".
                $row['itemDescription']."</td><td>
                <input type='checkbox'></input></td></tr>";  
    }  
    echo $output;  
}
else{
    $_POST["businessUnit"] = "";
}
?>

to handle undefined index error, $array[$key] != "" won't work, you have to use isset() first before not equal to blank check. See the solution below, it may works.

if (isset($_POST["businessUnit"]))
        {
            if ($_POST["businessUnit"] != '')
            {
                if (isset($_POST["productGroup"]) && $_POST["productGroup"] != '') //first undefined index
                {
                    $sql = "SELECT * FROM item WHERE businessUnit =         '" . $_POST["businessUnit"] . "' and productGroup = '" . $_POST["productGroup"] . "'";
                } else if (isset($_POST["deviceType"]) && $_POST["deviceType"] != '') //second undefined index
                {
                    $sql = "SELECT * FROM item WHERE businessUnit = 
'" . $_POST["businessUnit"] . "' and productGroup = '" . $_POST["deviceType"] . "'";
                } else if (isset($_POST["productGroup"]) && $_POST["productGroup"] != '' && isset($_POST["deviceType"]) && $_POST["deviceType"] != '') //third undefined index error
                {
                    $sql = "SELECT * FROM item WHERE businessUnit = '" . $_POST["businessUnit"] . "' and productGroup = '" . $_POST["productGroup"] . "' and deviceType = '" . $_POST["deviceType"] . "'";
                } else
                {
                    $sql = "SELECT * FROM item WHERE businessUnit = '" . $_POST["businessUnit"] . "'";
                }
            } else
            {
                $sql = "SELECT * FROM item";
            }
            $result = sqlsrv_query($connect, $sql);
            while ($row = sqlsrv_fetch_array($result))
            {
                $output .= "<tr><td>" .
                        $row['businessUnit'] . "</td><td>" .
                        $row['productGroup'] . "</td><td>" .
                        $row['deviceType'] . "</td><td>" .
                        $row['serialNumber'] . "</td><td>" .
                        $row['location'] . "</td><td>" .
                        $row['condition'] . "</td><td>" .
                        $row['itemDescription'] . "</td><td>
                <input type='checkbox'></input></td></tr>";
            }
            echo $output;
        } else
        {
            $_POST["businessUnit"] = "";
        }
    }