How to set a simple jQuery hide/show function to show all before hide?

I have a drop down selection, once it select it will show the div where the value and class share the same name and hide all else.

How can I have it so before you select it’ll show all and only hide after you select a option ?

EDIT I have updated the code snippet to run, have built in markup however on my local its php.

Revised question

how can I adjust my jQuery to show all, when none are selected and the selection is on default.

var $j = jQuery.noConflict();
        $j(document).ready(function () {
            $j("select").change(function () {
                $j(this).find("option:selected").each(function () {
                    var optionValue = $j(this).attr("value");
                    if (optionValue) {
                        $j(".career_div").not("." + optionValue).hide();
                        $j("." + optionValue).show();
                    } else {
                        $j(".career_div").hide();
                    }
                });
            })
        });

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select>
  <option value="all">Default</option>
    <option value="one">Intelligence (2)</option>
  <option value="two">Engineering (2)</option>
  <option value="three">Marketing (0)</option>
</select>

<section class="py-3 section-grey">
  <div class="container">
    <div class="row">
      <div class="col-12 one career_div">
        <strong>Intelligence</strong>
      </div>
      <div class="col-12 one career_div">
        <strong>Intelligence</strong>
      </div>
      <div class="col-12 two career_div">
        <strong>Engineering</strong>
      </div>
      <div class="col-12 line-grey two career_div">
        <strong>Engineering</strong>
      </div>
    </div>
  </div>
</section>

Answer

You can make a couple of changes:

use $(this).val() (this=select) to get the value= from the select option, no need to find option:selected and loop as .val() does this for you.
check for “all” then don’t apply the filter
as noted in the comments, “one”/”two” were around the wrong way (no impact on functionality, just makes it look like it’s wrong)

Updated snippet:

$(document).ready(function($) {
  $("select").change(function() {
    var opt = $(this).val();
    if (opt === "all") {
      $(".career_div").show();
    } else {
      $(".career_div").hide();
      $("." + opt).show();
    }   
  })
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select>
  <option value="all">Default</option>
  <option value="two">Engineering (2)</option>
  <option value="one">Intelligence (2)</option>
  <option value="three">Marketing (0)</option>
</select>

<section class="py-3 section-grey">
  <div class="container">
    <div class="row">
      <div class="col-12 one career_div">
        <strong>Intelligence</strong>
      </div>
      <div class="col-12 one career_div">
        <strong>Intelligence</strong>
      </div>
      <div class="col-12 two career_div">
        <strong>Engineering</strong>
      </div>
      <div class="col-12 line-grey two career_div">
        <strong>Engineering</strong>
      </div>
    </div>
  </div>
</section>

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Answer