How to optimize this IF MySQL query - mysql

Question

This MySQL query takes over 6 minutes to run. The bottleneck is the last piece which generates the "Activity Completed" column. The reason I've written it as a subquery is because if the itemtype = "course" I need to find the MAX date of all other item types for each user in each class, not the MAX date of the whole table column. I'm sure there's a much better way, but I don't know what it is. Can anyone help me?
SELECT DISTINCT
u.firstname AS 'First' , u.lastname AS 'Last', c.fullname AS 'Course',
IF (gi.itemtype = 'course',
CONCAT(c.fullname," COURSE TOTAL"),
gi.itemname) AS 'Item Name',
IFNULL(CONCAT(ROUND(gg.finalgrade,2),'/',ROUND(gi.grademax,2)), '0')
AS 'Points Earned',
IFNULL(CONCAT(ROUND((gg.finalgrade/gi.grademax),2)*100,'%'), '0%')
AS 'Percentage',
(SELECT
IF(`Percentage`>=90, 'A',
IF(`Percentage`<90 AND `Percentage`>=80, 'B',
IF(`Percentage`<80 AND `Percentage`>=70, 'C',
IF(`Percentage`<70 AND `Percentage`>=60, 'D', 'F')))))
AS 'Grade',
IF(gi.itemtype = 'course',
(SELECT
FROM_UNIXTIME(GREATEST(MAX(gg.timemodified), MAX(gg.timecreated)),
'%m/%d/%Y')
FROM mdl_course c
JOIN mdl_grade_items gi
JOIN mdl_grade_grades gg
WHERE gg.userid=u.id
AND gi.courseid=c.id),
CASE
WHEN gg.timecreated IS NULL AND gg.timemodified IS NULL THEN '-'
WHEN gg.timecreated IS NULL AND gg.timemodified IS NOT NULL THEN FROM_UNIXTIME(gg.timemodified, '%m/%d/%Y')
WHEN gg.timecreated IS NOT NULL AND gg.timemodified IS NULL THEN FROM_UNIXTIME(gg.timecreated, '%m/%d/%Y')
WHEN gg.timecreated<=gg.timemodified THEN FROM_UNIXTIME(gg.timemodified,'%m/%d/%Y')
END)
AS 'Activity Completed'
FROM mdl_user u
JOIN mdl_user_enrolments ue on ue.userid=u.id
JOIN mdl_enrol e ON e.id=ue.enrolid
JOIN mdl_course c on c.id = e.courseid
JOIN mdl_context AS ctx ON ctx.instanceid = c.id
JOIN mdl_role_assignments AS ra ON ra.contextid = ctx.id
JOIN mdl_role AS r ON r.id = e.roleid
JOIN mdl_grade_items AS gi ON gi.courseid=c.id
LEFT JOIN mdl_grade_grades AS gg ON gg.userid=u.id
AND gg.itemid=gi.id
WHERE ue.status='0'
AND gi.hidden = '0'
AND gi.itemtype <> 'category'
AND gg.excluded = '0'
OR gg.excluded IS NULL
ORDER BY u.lastname, c.fullname, gi.itemtype DESC, gi.sortorder ASC
I want the output to look like this:
First, Last, Course, Item Name, Points Earned, Percentage, Grade, Activity Completed
Test, Student1, Class1, Assignment 1, 10/10, 100%, A, 1/2/2013
Test, Student1, Class1, Assignment 2, 15/15, 100%, A, 3/4/2013
Test, Student1, Class1, Assignment 3, 0/15, 0%, F, -
Test, Student1, Class1, Class1 COURSE TOTAL, 25/40, 63%, D, 3/4/2013
Test, Student1, Class2, Assignment1, 20/30, 66%, D, 2/12/2013
Test, Student1, Class2, Assignment2, 1/5, 20%, F, 1/31/2013
Test, Student1, Class2, Class2 COURSE TOTAL, 21/35, 60%, D, 2/12/2013
Test, Student2, Class1, Assignment1, etc...
This is what I get when I prepend EXPLAIN to the query.
I couldn't copy the column headers. They are:
id, select_type, table, type, possible_keys, key, key_len, ref, rows, extra
1 PRIMARY u ALL PRIMARY 449 Using temporary; Using filesort
1 PRIMARY ue ref mdl_userenro_enruse_uix,mdl_userenro_enr_ix,mdl_userenro_use_ix mdl_userenro_use_ix 8 skol_dev.u.id 1 Using where
1 PRIMARY e eq_ref PRIMARY,mdl_enro_cou_ix PRIMARY 8 skol_dev.ue.enrolid 1
1 PRIMARY r eq_ref PRIMARY PRIMARY 8 skol_dev.e.roleid 1 Using index
1 PRIMARY c eq_ref PRIMARY PRIMARY 8 skol_dev.e.courseid 1
1 PRIMARY ctx ref PRIMARY,mdl_cont_ins_ix mdl_cont_ins_ix 8 skol_dev.c.id 1 Using where; Using index
1 PRIMARY gi ref mdl_graditem_itenee_ix,mdl_graditem_cou_ix mdl_graditem_cou_ix 9 skol_dev.e.courseid 7 Using where
1 PRIMARY ra ref mdl_roleassi_con_ix mdl_roleassi_con_ix 8 skol_dev.ctx.id 4 Using index
1 PRIMARY gg eq_ref mdl_gradgrad_useite_uix,mdl_gradgrad_ite_ix,mdl_gradgrad_use_ix mdl_gradgrad_useite_uix 16 skol_dev.u.id,skol_dev.gi.id 1 Using where
3 DEPENDENT SUBQUERY gg ref mdl_gradgrad_useite_uix,mdl_gradgrad_use_ix mdl_gradgrad_use_ix 8 skol_dev.u.id 6
3 DEPENDENT SUBQUERY c index PRIMARY mdl_cour_cat_ix 8 124 Using index; Using join buffer
3 DEPENDENT SUBQUERY gi ref mdl_graditem_cou_ix mdl_graditem_cou_ix 9 skol_dev.c.id 7 Using where; Using index