Joins with IS NULL

[Donovan]

".$prefix."_tl_students  contain all students.

 

".$prefix."_tl_session_grades contains all grades assigned to students.

 

A session is equivalent to a class on a single day.  The session is a "Team Learning" event where they all participate.

 

If a students misses a "Team Learning" session then they do not get a record written to the _tl_session_grades, however I still need them to be displayed showing no grades recorded for that session.  I thought the following would address what I need but I am not getting any results.  I need to see all students even the ones who do not have a grade recorded. 

 

$result = $db->sql_query("SELECT * FROM ".$prefix."_tl_session_grades a 
LEFT JOIN ".$prefix."_tl_students b ON (a.SOMS_KEY = b.SOMS_KEY) 
WHERE a.Session_ID = '$Session_ID' AND a.SOMS_KEY IS NULL");

 

Where have I gone wrong?

[Barand]

It needs to be

 

students LEFT JOIN session_grades

 

as you always want all students