How to create sub menu in side menu in html & css

Question

I have 1 side menu in my main page of website. And some of the list have have sub menu. And on hover i want to display it. Firstly i am display it none and then on display it on hover but it is not displaying.

#menu {
  width: 15%;
  height: 100%;
  background: #424242;
  color: white;
  float: left;
}

#menu ul {
  margin: 0;
  padding: 0;
}

#menu li {
  list-style: none;
  font-size: 20px;
  padding: 15px 20px;
  cursor: pointer;
}

#menu li:hover {
  background-color: #90CAF9;
}

#menu li.active {
  background-color: #2196F3;
}

#menu li ul {
  display: none;
}

#menu li:hover ul {
  display: block;
  position: absolute;
  left:100%
}

<div id="menu">
  <ul>
    <li onClick="Dashboard();">Dashboard</li>
    <li>Employee &gt;
      <ul>
        <li>Add Employee</li>
        <li>Employee View</li>
      </ul>
    </li>
    <li>Leave</li>
    <li>Salary</li>
    <li>Change Password</li>
  </ul>
</div>

Answer 1

#menu {
  width: 150px;
  height: 100%;
  background: #424242;
  color: white;
  float: left;
}

#menu ul {
  margin: 0;
  padding: 0;
}

#menu li {
  list-style: none;
  font-size: 20px;
  padding: 15px 20px;
  cursor: pointer;
}

#menu li:hover {
  background-color: #90CAF9;
}

#menu li.active {
  background-color: #2196F3;
}

#menu ul li ul {
  display: none;
}

#menu ul li.submenu {
  position: relative
}

#menu ul li.submenu ul {
    position: absolute;
    left: 150px;
    width: 200px;
    top: 0;
    background: #333
}

#menu ul li.submenu:hover ul {
  display: inline-block
}

<div id="menu">
  <ul>
    <li onClick="Dashboard();">Dashboard</li>
    <li class="submenu">Employee &gt;
      <ul>
        <li>Add Employee</li>
        <li>Employee View</li>
      </ul>
    </li>
    <li>Leave</li>
    <li class="submenu">Salary
      <ul>
        <li>Add Employee</li>
        <li>Employee View</li>
      </ul>
    </li>
    <li>Change Password</li>
  </ul>
</div>

Also Add .submenu class to submenu dropdown parent li.