While inside While from different MySQL table - php

Question

i have 2 tables:
Cat
id,cat_name
SubCat
id, subcat_name, under_cat
i want to make a list that loads the sub category into the correct Category
the problem is with the second loop, i cant make it work
what am i doing wrong?
<?
$query = "SELECT * FROM `Cat`";
$result = $mysqli->query($query);
while ($row = $result->fetch_assoc()){
echo '<li><a href="#">'
. $row['cat_name'] .
'</a><ul>';
$query2 = "SELECT * FROM `SubCat` WHERE under_cat = '". $row['cat_id'] ."'";
$result2 = $mysqli->query($query2);
while ($row2 = $result->fetch_assoc()){
echo '<li><a href="#">'
. $row2['subcat_name'] .
'</a></li>';
}
echo '</ul></li>';
}
?>
EDIT:
two lists from same category
this is the code:
<?
$query = "SELECT * FROM Cat LEFT OUTER JOIN SubCat ON SubCat.under_cat = Cat.cat_id;";
$result = $mysqli->query($query);
while ($row = $result->fetch_assoc()){
echo '<li><a href="#">'
. $row['cat_name'] .
'</a><ul>';
if($row['cat_id'] === $row['under_cat']){
echo '<li>'.$row['subcat_name'].'</li>';
}
echo '</ul></li>';
}
?>

Answer 1

I hate doing SELECT * because of their maintenance problems, but to work with what you did, just do a JOIN:
"SELECT * FROM Cat LEFT OUTER JOIN SubCat ON SubCat.under_cat = Cat.cat_id;"
This will return the result set you want, but you will need to modify your PHP in response to create the list appropriately, something like this:
if($row['cat_id'] === $id){
echo '<li>'.$row2['subcat_name'].'</li>';
} else {
echo '</ul></li><li>'.$row['cat_name'].'<ul>';
iF(!empty($row['subcat_name'])){
echo '<li>'.$row['subcat_name'].'</li>';
}
}
$id = $row['cat_id'];
This will add the items to the list if it is the same as the previous entry, else it will start a new list. The !empty thing is a quick attempt at handling NULLs, as in where there were no matches brought back from SubCat but you had entries in Cat. If this would never be the case, you can eliminate the if statement and convert your query to this:
"SELECT * FROM Cat INNER JOIN SubCat ON SubCat.under_cat = Cat.cat_id;"
Why go through all this effort you ask? Because this will provide the result set you want while only querying once. The way you had it structured prior would query a bazillion times, which kills the server and greatly reduces performance.
Hopefully this helps both your problem and you're overall implementation knowledge.