HI!

Select doesn't have placeholder attribute. But you can do placeholder by yourself :D Sample code should look like this:

<select class="mdb-select md-form">
    <option value="" disabled selected>Choose your option</option>
    <option value="1">Option 1</option>
    <option value="2">Option 2</option>
    <option value="3">Option 3</option>
 </select>

First option is your placeholder. It value should be empty and it should have disabled and selected attributes. Between <option></option> tags you have to put your placeholder text.

And voila! You have your default option, that can't be selected second time, when you choose another.

You can see how our material select works here: https://mdbootstrap.com/docs/jquery/forms/select/#basic