table_1
+----------+-----------+--------------+
| category | Spec_Name | Spec_Value   |
+----------+-----------+--------------+
| Engine   |    x      | 111111111111 |
+----------+-----------+--------------+
| Engine   |    y      | 222222222222 |
+----------+-----------+--------------+

table_2
+----------+-----------+--------------+
| category | Spec_Name | Spec_Value   |
+----------+-----------+--------------+
| Engine   |    x      | 111111111111 |
+----------+-----------+--------------+

So, the Spec_Name.y wouldn't exist in table_2.


In that case, I'd like to show this for a user:

+---------+----------------+----------------+
|Spec_Name| car1.Spec_Value|car2.Spec_Value |
+---------+----------------+----------------+
|  x      | 111111111111   |        -       |
|  y      |      -         |    55555565    |
|  z      | 333333333333   |    77777777    |
etc.

function printCarData($conn, $table){
  $query = "SELECT  Spec_Name, Spec_Value, SUBSTRING(Spec_Value, 1, 20), TechSpecCategoryName  FROM ".$table;
  if($result = mysqli_query($conn, $query)){
    echo"<ul>";
    while($row = mysqli_fetch_assoc($result)){

     if($query != $row["TechSpecCategoryName"]){

        echo "<li>"."<br>"."</li>";
      }
      echo "<li>".$row["SUBSTRING(Spec_Value, 1, 20)"]."</li>";

      $query = $row["TechSpecCategoryName"];
    }
    echo"</ul>";
  }
}

printCarData($link, $table_name)