Check if "tag" is available and show it in a SELECT

I realize a system that basically is a logs of loan equipment. What I intend you to do is that users who need a computer to loan make a record, which is necessary to validate if the equipment is available or not for its use, and only appear in SELECT the equipment available .

My main table prestamo is composed of foreign keys. I have fields type NULL since those fields have to see, or rather, they are modified or filled by the administrator when the user returns the equipment (in another loan edition form).

Here the structure of my table prestamo :

id_prestamo      int(15)          Primaria    
id_etiqueta      int(15)          FK         UNIQUE 
fecha1           date
hora1            time
id_usuario       int(15)          FK
id_departamento  int(15)          FK
id_entrega       int(15)          FK          
fecha2           date                         NULL
hora2            time                         NULL
id_ingeniero     int(15)          FK          NULL

The validation that I need you to perform is on the field id_etiqueta and on id_ entrega , which is in the table prestamo (mentioned above).

Structure table tag

 id_etiqueta    int(15)         Primaria
 descripcion    varchar(50)

Structure table entrega :

 id_entrega     int(15)         Primaria
 descripcion    varchar(50)

An example is:

If my field id_etiqueta = 1 ( 1 equals LAP-01 ) compare it with id_entrega = 1 ( 1 equals OCUPADO ).

Then NO will show that label in SELECT of id_etiqueta so that users can not see it.

This is so that they can not occupy it, until you change id_entrega = 2 ( 2 equals Disponible ), this is when it will appear again in the SELECT tag.

PHP Code:

<?php

 require("connect_db.php");

 if (
    false === empty($_POST['id_etiqueta'])
    && false === empty($_POST['fecha1'])
    && false === empty($_POST['hora1'])
    && false === empty($_POST['id_usuario'])
    && false === empty($_POST['id_departamento'])

) {
    /* Preparamos la consulta usando ? para insertar los valores */
    $stmt = $mysqli->prepare("
        INSERT INTO prestamo (
            id_etiqueta,
            fecha1,
            hora1,
            id_usuario,
            id_departamento       
        ) VALUES (
            ?,
            ?,
            ?,
            ?,
            ?
        )
    ");
    /* Comprobamos si hubo problemas con la preparación de la consulta */
    if ($stmt === false) {
        die($mysqli->error);
    }
    /* Asignamos las variables, no es necesario crear variables intermedias */
    $stmt->bind_param('sssss',
        $_POST['id_etiqueta'],
        $_POST['fecha1'],
        $_POST['hora1'],
        $_POST['id_usuario'],
        $_POST['id_departamento']   
    );
    /* Comprobamos si se pudo realizar la consulta */
    if($stmt->execute() === true) {
        $mensaje = "Registro agregado correctamente";
    } else {
        /* Comprobamos si hubo duplicidad de registro (clave duplicada) */
        if ($stmt->errno === 1062) {
            $mensaje = "Error:\nRegistro duplicado";
        } else {
            $mensaje = "Error al ingresar:\n" . $stmt->error;
        }
    }
    /* Enviamos al navegador de manera segura el mensaje de texto */
    echo '<script>alert(', json_encode($mensaje), ')</script>';
}
?>

Select id_etiqueta

Here I insert my database by ID, since it is a foreign key and I show it by description to the user:

<div class="form-group">
<label for="id_etiqueta" class="col-sm-2 control-label">Equipo Disponible:</label>
<?php
require("connect_db.php");

$query = "SELECT DISTINCT e.id_etiqueta, e.descripcion 
FROM etiqueta e
INNER JOIN prestamo p on p.id_etiqueta=e.id_etiqueta
WHERE p.id_entrega!=1";
$res = $mysqli->query($query);
$option = '';
while ($row = $res->fetch_assoc()){

    $option.="<option value=\"$row[id_etiqueta]\">$row[descripcion] </option>";   

}
?>


<div class="col-sm-10">


<select type="text" class="form-control" id="id_etiqueta" name="id_etiqueta" placeholder="Etiqueta" required>
<option value="-">Selecciona Equipo Disponible</option>
<?php echo $option; ?>
</select></div>