select
не является подходящим элементом для этой задачи, a dropdown
намного лучше подходит.
Нет способа по умолчанию для создания подменю, но его достаточно легко достичь.
$(document).ready(function() {
$('.dropdown-submenu a').on("click", function(e) {
$(this).parents('ul').find('.dropdown-submenu ul').hide();
$(this).next('ul').toggle();
e.stopPropagation();
e.preventDefault();
});
});
.dropdown-submenu {
position: relative;
}
.dropdown-submenu .dropdown-menu {
top: 0;
left: 100%;
margin-top: -1px;
}
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css" integrity="sha384-MCw98/SFnGE8fJT3GXwEOngsV7Zt27NXFoaoApmYm81iuXoPkFOJwJ8ERdknLPMO" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>
<div class="dropdown">
<button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenuButton" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
All South Africa
</button>
<ul class="dropdown-menu" aria-labelledby="dropdownMenuButton">
<li class="dropdown-submenu">
<a class="dropdown-item" href="#">Eastern Cape</a>
<ul class="dropdown-menu">
<li><a class="dropdown-item" href="#">Umtata / Mthatha</a></li>
<li><a class="dropdown-item" href="#">Willowmore</a></li>
<li><a class="dropdown-item" href="#">Willowvale</a></li>
</ul>
</li>
<li class="dropdown-submenu">
<a class="dropdown-item" href="#">Free State</a>
<ul class="dropdown-menu">
<li><a class="dropdown-item" href="#">Koppies</a></li>
<li><a class="dropdown-item" href="#">Kroonstad</a></li>
</ul>
</li>
</ul>
</div>