SQL query ORDER BY looping in PHP - php

Question

EDIT: Added the first SQL query.
A section of my website has two dropdown menus. All the options in both are populated using SQL queries. Dropdown#1 is a list of class sections (like A1 for example). Once the professor selects a section, Dropdown#2 is populated with the student ID's (like 1234567 for example).
Student information is found in table 1. Among this information is the 'professorName' column. In order to associate the student with a class section, I need to match 'professorName' column with an identical column found in table 2, because class sections are only found in table 2.
Till here everything works great, because at the end of my query I put ORDER BY student ID. However, two of the class sections are associated to two different professors. In order to deal with this issue, I used the following code to loop through each professor name.
$from site = $_POST['section'];
$query = ("SELECT professorName FROM Table 2 WHERE classSection='$fromsite'");
$NumberofProfessorNames = $objMSSQL->getAffectedRows();
echo $NumberofProfessorNames;
for ($j=0; $j<$NumberofProfessorNames; $j++)
{
$section= $query[$j][professorName];
$output = $objMSSQL->getTable("SELECT DISTINCT StudentID from table1 WHERE professorName='$section' ORDER BY StudentID");
for ($i=0; $i<$objMSSQL->getAffectedRows(); $i++)
{
echo "<option value='".$output[$i][studentID]."'>".$output[$i][studentID]."</option>";
}
}
The problem is that for the only two sections where this is even necessary (because there are two professorNames), since it is looping like this, it is ending up ordered like this in the dropdown#2:
1234567
2345678
3456789
4567890
1234123
2345765
3456999
4567000
My limited experience in programming is keeping me from understanding how I can fix this seemingly simple issue.
Thank you for your help.

Answer 1

Rather than loop over the professors and query table1 for each, join table1 and table2 in the second query and only query the database once. For example:
$query = [... FROM Table2...];
$NumberofProfessorNames = $objMSSQL->getAffectedRows();
echo $NumberofProfessorNames;
$output = $objMSSQL->getTable("
SELECT DISTINCT StudentID
from table1
join table2
on ...
WHERE [the same clause you used in $query]
ORDER BY StudentID"
);
for ($i=0; $i<$objMSSQL->getAffectedRows(); $i++)
{
echo "<option value='".$output[$i][studentID]."'>".$output[$i][studentID]."</option>";
}
It's more elegant (and almost certainly more efficient) than generating a WHERE IN clause.

Answer 2

Yu can do it this way:
$section = "('";
for ($j=0; $j<$NumberofProfessorNames; $j++)
{
$section.= $query[$j][professorName] . "','";
}
$section = substr($section, 0, -3) . ')'; //$section contains ('prof1','prof2')
$output = $objMSSQL->getTable("SELECT DISTINCT StudentID from table1 WHERE professorName IN $section ORDER BY StudentID");
for ($i=0; $i<$objMSSQL->getAffectedRows(); $i++)
{
echo "<option value='".$output[$i][studentID]."'>".$output[$i][studentID]."</option>";
}
that is querying for all your professors in just one sql with IN() syntax.
UPDATE: I've just noted you use sql server instead of mysql, so I've changed the IN() syntax a bit and change the link to the sql server help docs.

Answer 3

It sounds like your tables aren't normalized. Good form would have a sections table, a students table, and a professors table. Information in each table should be specific to the table's topic.
students
student_id
student_last_name
student_first_name
student_address
etc
sections
section_id
section_name - multiple sections can tend to have the same name but differing content
section_description
section_year - sections can change from year to year
faculty
faculty_id
faculty_name - this is not a key field, more than one person can have the same name.
faculty_address
faculty_type - adjunct, fulltime, etc.
You would then have relational tables so you can associate professors with sections and students with sections.
faculty_2_sections
f2s_id
faculty_id
section_id
student_2_sections
s2s_id
student_id
section_id
This makes it super simple because if a student is logged in, then you already have their student id. If it's a professor, you already have their faculty_id
If you're pulling for students, your sql might look like this:
$sql = "select * from students s,sections sc,faculty f,faculty_2_sections f2s,student_2_sections s2s where student_id='$student_id' and s2s.student_id=s.student_id and s2s.section_id=sc.section_id and f2s.faculty_id=f.faculty_id and f2s.section_id=s2s.section_id";
If you're pulling for faculty you would do this:
$sql = "select * from students s,sections sc,faculty f,faculty_2_sections f2s,student_2_sections s2s where faculty_id='$faculty_id' and f2s.faculty_id=f.faculty_id and f2s.section_id=s2s.section_id and s2s.section_id=sc.section_id and s2s.student_id=s.student_id";
You can then pull a list of sections to populate the section_ids pull-down to only show students or faculty for a specific section.