From: http://moodle.org/mod/forum/discuss.php?d=138680 by Ohad Kravchick - Wednesday, 30 December 2009, 09:10 PM

Ok. I finally was able to find the problem.

I could not get permissions to access the server. However, I had created the same environment on my own pc; this is important as the problem seems OS/DB related. The server is CentOS (CentOS 4.5; Linux Kernel 2.6.9-55.EL), with NGINX (0.8.30) with PHP (5.2.9) through FastCGI (spawn-fcgi-1.6.3) and mySQL (4.1.20); it uses Moodle 1.9.7 build 20091126.

The problem starts in /mod/quiz/locallib.php at quiz_has_feedback (line 345). It creates a query like: select * from quiz_feedback WHERE quizid=1 AND NOT feedbacktext='';

In /lib/dmllib.php In sql_isnotempty (line 1932), Moodle try to compare using "return ' ( NOT ' . sql_isempty($tablename, $fieldname, $nullablefield, $textfield) . ') ';"

The query returns no rows no matter what my table contains.

See:

mysql> select * from quiz_feedback WHERE quizid=1;
--------------------------------------

--------------------------------------

--------------------------------------
2 rows in set (0.00 sec)

mysql> select * from quiz_feedback WHERE quizid=1 AND NOT feedbacktext='';
Empty set (0.00 sec)

mysql> select * from quiz_feedback WHERE quizid=1 AND feedbacktext='Good';
--------------------------------------

--------------------------------------

--------------------------------------
1 row in set (0.00 sec)

mysql> select * from quiz_feedback WHERE quizid=1 AND NOT feedbacktext='Good';
Empty set (0.00 sec)

It appears that NOT doesn't interact well with = comparizon for string.

Instead, this should be used:

mysql> select * from quiz_feedback WHERE quizid=1 AND feedbacktext<>'';
--------------------------------------

--------------------------------------

--------------------------------------
2 rows in set (0.00 sec)

or strcmp, or "not like", etc. Perhaps only for mysql.

This looked like the sort of thing you should know about Eloy.