How to display data linked by foreign key to mapping table.

Question

I have been using php my admin to create a database and the time has come to display my data in my webpage.

I am struggling to display data linked to my mapping table with a foreign key.

The tables I have are:

Subcategory

subcategory_id (pk) | Subcategory name

Subsubcategory

subsubcategory_id (pk) | subsubcategory_name

Sub_subsub (mapping table)

id (pk) | s_id (fk) | ss_id (fk)

s_id and ss_id are foreign keys to the respective tables.

Here is the code I am playing with without any joy. Im still quite new to php

$dbc = mysql_connect($db_host,$db_user,$db_pass);
$sdb = mysql_select_db($db_database);

$query = "SELECT s_id as subcategory_id, ss_id as subsubcategory_id FROM sub_subsub";
$result = mysql_query($query, $dbc)
or die (mysql_error($dbc));

while($row = mysql_fetch_array($result)) {

$subcat = $row["s_id"];
$subsubcat = $row["ss_id"];

echo "<li>$subcat <span>$subsubcat</span></li>";
}

Im probably missing something quite obvious but I've been searching and cant find anything.

Answer 1

Well, in your query you give the s_id field new name (or alias) subcategory_id (the s_id as subcategory_id part in the query) but in your code you still try to access it by the name s_id . Try

$subcat = $row["subcategory_id"];

Or drop the alias from query. Similar stuff with ss_id field.

Try something like

SELECT 
  SC.Subcategory_name ,
  SS.subsubcategory_name
FROM Sub_subsub M
  JOIN Subcategory SC ON(SC.subcategory_id = M.s_id)
  JOIN Subsubcategory SS ON(SS.subsubcategory_id = M.ss_id)

to get name pairs instead of ID pairs.

BTW your table design looks somewhat suspicious... usually this kind of category hierarcy is done with only one table, something like

TABLE TAB_Category (
  UID        PRIMARY KEY,
  Parent     FK REFERENCES TAB_Category(UID) ON UPDATE CASCADE,
  Name       
);

where root items do have Parent = NULL.