从PHP和SQL加入多个表，并为每个循环返回一个字符串

Question

I'm trying to build a workout system. The user selects a piece of equipment from a multiselect dropdown. Each piece of equipment has associated exercises. When a piece of equipment is selected, I need to select a button which runs a query and joins the tables to get an associated exercise and display these exercises in a random order.

The below query gets the selected equipment from the DB from a selected multiselect.

<form method="post">
    <select name="selected[]" class="selectpicker" multiple data-live-search="true">
        <option><?php echo $equipment; ?></option>
    </select>
    <br/>
    <br/>
    <button type="submit" name="submit" class="btn btn-outline-success btn-sm"> Confirm </button>
</form>

      <?php
      //check if form is submitted successfully
      if(isset($_POST["submit"]))
      {
          //check if any option is selected
          if(isset($_POST["selected"]))
          {
              // retrive each selected option
              foreach ($_POST['selected'] as $selected)

                  print "You selected $selected<br/>";
                  print "Select the Shuffle button to get workout";
              }
              else
                  echo "Select an option first";
          }

I have tried the below to join the tables and display an exercise

// retrieve each selected option

foreach ($_POST['selected'] as $selected)
"GET ExerciseID FROM `ExerEquip`
JOIN Exercise
ON Exercise.ExerciseID =   ExerEquip.ExerciseID
WHERE `EquipmentName`= selected
order by rand()
LIMIT 1;";
    print "You selected $selected<br/>";
    print "Select the Shuffle button to get workout";