Using results from one MySQL query to insert into another query

Question

I am trying to use PHP and MySQL to automatically generate a list like this.

Subject #1

• Resource #3
• Resource #5
• Resource #12

Subject #2

• Resource #7
• Resource #4
• Resource #1

etc.

Here are the tables:

eresources

• erid
• etitle

subjects

• sid
• stitle

subjectmap

• sid
• erid

Here is the code that successfully gives me all the subjects in alphabetical order:

$subjectQuery = "SELECT * FROM subjects WHERE sid != 17 ORDER BY stitle ASC";
$subjectResult = $mysqli->query($subjectQuery);

while ($subjectArray = $subjectResult->fetch_assoc()) { 
print "<h5 class='subcategory'>" . $subjectArray['stitle'] . "</h5>";
print "<div class='inner'>  

// Need a list of resources that match each subject ID (sid) here!

</div>";
}

Now, here is the code that successfully gives me all the e-resources with a fixed subject ID:

$getERBySubjectQuery = " SELECT erid FROM subjectmap WHERE sid=11 ";
$getERBySubjectResult = $mysqli->query($getERBySubjectQuery);
if($getERBySubjectResult && $getERBySubjectResult->num_rows >= 1){
 while($getERBySubjectArray = $getERBySubjectResult->fetch_assoc() ){
        $query = " SELECT * FROM eresources WHERE erid = " .$getERBySubjectArray['erid']. " ORDER BY ertitle ASC ";
    $result = $mysqli->query($query);

    if($result && $result->num_rows >= 1){
       while($array = $result->fetch_assoc() ){
          print("<a href=\"" . $array['link'] . "\">" .   "<h5 class='subcategory'>" . $array['ertitle'] . "<div class='accessnote'>"   .  $array['access']    . "</div></h5></a>");

 }}}}

Naturally, I am not inclined to duplicate the same stanza for each subject ID (sid). I want the list of subjects to be generated automatically and the list of resources that have that subject to be generated automatically as well.

Basically I need to feed the sid from the first stanza into the second stanza for each subject.

Answer 1

You can't do this in MySQL without executing n+1 queries where n is the number of subjects.

You could just use a JOIN and some clever sorting.

SELECT
    subject.stitle, eresource.etitle
FROM subject
    LEFT JOIN subjectmap ON subject.id = subjectmap.sid
    LEFT JOIN eresource ON eresource.id = subjectmap.erid
ORDER BY subject.stitle, eresource.etitle

You will get a result like:

+------------+--------------+
| stitle     | etitle       |
+------------+--------------+
| Subject #1 | Resource #3  |
| Subject #1 | Resource #5  |
| Subject #1 | Resource #12 |
| Subject #2 | Resource #1  |
| Subject #2 | Resource #4  |
| Subject #2 | Resource #7  |
+------------+--------------+

Answer 2

By using a WHERE clause instead, I think Frits van Campen's query could be more human readable, especially for a beginner :

SELECT subject.stitle, eresource.etitle
FROM subject,subjectmap,eresource 
WHERE subject.id = subjectmap.sid
  AND eresource.id = subjectmap.erid
ORDER BY subject.stitle, eresource.etitle