Joining mysql tables

I am trying to join tables for a search form, and when I do the search exports the result x3 the output could you guys give me a hand please?

   $raw_results = mysqli_query($db,"SELECT * FROM inventoryuser, inventorypc
      WHERE (`firstname` LIKE '%".$query."%')
      OR (`lastname` LIKE '%".$query."%')
      OR (`department` LIKE '%".$query."%')
      ")
      or die(mysqli_error($db));

The output

I need to search inventorypc for pc_model and pc_serial that's under a different table.

My tables are:

CREATE TABLE `inventoryuser` (
  `userid` INT(11) NOT NULL AUTO_INCREMENT,
  `firstname` VARCHAR(30) NOT NULL,
  `lastname` VARCHAR(30) NOT NULL,
  `department` VARCHAR(30) NOT NULL,
PRIMARY KEY (`userid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

CREATE TABLE `inventorypc` (
  `pcid` INT(11) NOT NULL AUTO_INCREMENT,
  `pc` VARCHAR(30) NOT NULL,
  `jcid` VARCHAR(30) NOT NULL,
  `job_number` VARCHAR(30) NOT NULL,
  `product_id` VARCHAR(30) NOT NULL,
  `pc_model` VARCHAR(30) NOT NULL,
  `pc_serial` VARCHAR(30) NOT NULL,
  `software` VARCHAR(30) NOT NULL,
PRIMARY KEY (`pcid`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

And I the code I have is

  $query = $_GET['query'];
  // gets value sent over search form

  $min_length = 2;
  // you can set minimum length of the query if you want

  if(strlen($query) >= $min_length){ // if query length is more or equal minimum length then

      $query = htmlspecialchars($query);
      // changes characters used in html to their equivalents, for example:  0){ // if one or more rows are returned do following

          echo "
              
                
                  
                    
                    First Name
                    Last Name
                    Company Laptop
                    Company Mobile
                    Department
                    Actions
                  
                
          ";

              while($results = mysqli_fetch_array($raw_results)){
              // $results = mysql_fetch_array($raw_results) puts data from database into array, while it's valid it does the loop


              echo "
                    
                    
                      
                      ".$results['firstname']."
                      ".$results['lastname']."
                      ".$results['pc']."
                      ".$results['mobile']."
                      ".$results['department']."
                      
                        View
                        Edit
                        Delete
                      
                    
                  
                ";
              }

          }
          else{ // if there is no matching rows do following
              echo "No results";
          }

      }
      else{ // if query length is less than minimum
          echo "Minimum length is ".$min_length;
      }

      echo "
      ";