我不能再插入一个刚刚空置的摊位上的另一个租户，

Question

嗨大家我有一个监控系统，它可以监控进来的租户并在一个摊位出来。如果您是第一次在该摊位上插入租户，INSERT可正常工作，但在该租户未租赁并使该租户状态无效后，他/她离开的那个摊位现在可用。之后，我不能在那个摊位上插入租户。

这是我的代码：

<?php
include ('dbcon.php');

if (isset($_POST['add'])){
$tenant_fname = $_POST['tenant_fname'];
$tenant_mname = $_POST['tenant_mname'];
$tenant_lname = $_POST['tenant_lname'];
$tenant_address = $_POST['tenant_address'];
$tenant_contact = $_POST['tenant_contact'];
$stall_id = $_POST['stall_id'];
$stall_name = $_POST['stall_name']; 
$stall_category = $_POST['stall_category'];

$q1 = $conn->prepare("SELECT * FROM tenant WHERE stall_id = ? ");
$q1->execute(array($stall_id));
$c = $q1->rowCount();

if (!$stall_id){
header('location:pages/tenants.php');
}else{

if ($c > 0){
    header('location:pages/tenants.php');
}else{

    $add = $conn->prepare("INSERT INTO tenant (rate_id,tenant_fname, tenant_mname, tenant_lname, tenant_address, tenant_contact, stall_id, stall_name, stall_category)
        VALUES (?,?,?,?,?,?,?,?,?) ");
    $add->execute(array(1,$tenant_fname,$tenant_mname,$tenant_lname,$tenant_address,$tenant_contact,$stall_id,$stall_name,$stall_category));
    $count = $add->rowCount();
    $tenant_id = $conn->lastInsertId();
    $date_today = date('F d, Y');

    if ($count > 0){
        $add1 = $conn->prepare("INSERT INTO tenant_status (tenant_id, date_started, date_ended, tenant_status) VALUES(?,?,?,?) ");
        $add1->execute(array($tenant_id,$date_today,0,1));

        $add2 = $conn->prepare("INSERT INTO rent (tenant_id,stall_id,rent_status) VALUES (?,?,?)");
        $add2->execute(array($tenant_id,$stall_id,1));

        $add3 = $conn->prepare("INSERT INTO receipt(tenant_id) VALUES (?)");
        $add3->execute(array($tenant_id));

    }
    header('location:pages/tenants.php');
}
}

}
?>

BTW，值0表示租户无效，1表示有效。