php - Group by 'column name' doesn't seem to work -

i trying fetch category name has been saved multiple times before. fetch name need group name doesn't display multiple times, tried doing in way mentioned before , have failed far. i'm sure doing horribly wrong here , not able find why not able group it. appreciated. thanks.

$valsubcat=mysqli_query($conn,"select categoryid                                 subcategorymaster                                 subcatid=$valitem1[0]");  while($valsubcat1=mysqli_fetch_array($valsubcat)) {     $valcat=mysqli_query($conn,"select categoryname                                  categorymaster                                  categoryid=$valsubcat1[0]                                  group categoryname");     while($valcat1=mysqli_fetch_array($valcat))      {         echo "<tr><td>".$valcat1[0]."</td></tr>";        } } 

you can reduce 1 query using join

$sql = "select distinct scm.categoryid, cm.categoryname         subcategorymaster scm             left join categorymaster cm on cm.categoryid = scm.categoryid         scm.subcatid = {$valitem1[0]}");