服务器端数据表中的内部连接4表

我有代码PHP显示数据表中的数据，但这个PHP(服务器端)不工作。哪些数据表不能使用内连接?如果可以，如何修复我的代码?再一次，如何通过内连接在数据表中主动搜索数据。非常感谢:

<?php
require_once '../config/config.php'; // Use require. Can not use INCLUDE function
// storing  request (ie, get/post) global array to a variable  
$requestData = $_REQUEST;
$columns = array(
// datatable column index  => database column name
    0 => 'id_hanca',
    1 => 'id_detail_po',
    2 => 'ukuran',
    3 => 'jumlah_hanca',
    4 => 'status_hanca',
    5 => 'id_user',
    6 => 'id_vendor'
);
// getting total number records without any search
$sql = "SELECT po_detail.id_detail_po, hanca.id_hanca, hanca.ukuran, hanca.jumlah_hanca, user.name_usr, vendor.nama_vendor";
$sql.= "FROM hanca";
$sql.= "INNER JOIN po_detail ON po_detail.id_detail_po = hanca.id_detail_po";
$sql.= "INNER JOIN user ON user.name_usr = hanca.id_user";
$sql.= "INNER JOIN vendor ON vendor.id_vendor = hanca.id_vendor";
$query = $db->query($sql);
$totalData = $query->num_rows;
$totalFiltered = $totalData;  // when there is no search parameter then total number rows = total number filtered rows.
$sql = "SELECT * ";
$sql.=" FROM hanca WHERE 1=1";
if (!empty($requestData['search']['value'])) {   // if there is a search parameter, $requestData['search']['value'] contains search parameter
    $sql.=" AND ( id_hanca LIKE '%" . $requestData['search']['value'] . "%' ";
    $sql.=" OR id_detail_po LIKE '%" . $requestData['search']['value'] . "%' ";
    $sql.=" OR ukuran LIKE '%" . $requestData['search']['value'] . "%' )";
}
$query = $db->query($sql);
$totalFiltered = $query->num_rows; // when there is a search parameter then we have to modify total number filtered rows as per search result. 
$sql.=" ORDER BY " . $columns[$requestData['order'][0]['column']] . " " . $requestData['order'][0]['dir'] . "  LIMIT " . $requestData['start'] . " ," . $requestData['length'] . " ";
$query = $db->query($sql);
$data = array();
$no = 1;
foreach ($query as $row) {
    
    
    
    //Nama User
//    $qry = $db->query("SELECT name_usr FROM user WHERE id_usr='" . $row['id_user'] . "'");
//    $nm_user = $qry->fetch_assoc();
    
    $nestedData = array();
    $nestedData[] = $no++;
    $nestedData[] = $row['id_detail_po'];
    $nestedData[] = $row['ukuran'];
    $nestedData[] = $row['jumlah_hanca'];
    $nestedData[] = $row['name_usr'];    
    $nestedData[] = $row['nama_vendor'];
    // Input Hiddden to include value for update cart
    // Add html button for action
    $nestedData[] = "<a href='#' class='" btn btn-info btn-xs btn-flat'" data-toggle='"tooltip'" data-placement='"top'" title='"Detail Belanja'"><span class='"glyphicon glyphicon-search'"></span> Detail</a>";
    $data[] = $nestedData;
}
$json_data = array(
    "draw" => intval($requestData['draw']), // for every request/draw by clientside , they send a number as a parameter, when they recieve a response/data they first check the draw number, so we are sending same number in draw. 
    "recordsTotal" => intval($totalData), // total number of records
    "recordsFiltered" => intval($totalFiltered), // total number of records after searching, if there is no searching then totalFiltered = totalData
    "data" => $data   // total data array
);
echo json_encode($json_data);  // send data as json format