Analytics and reporting

HI,

I asked in another forum before I realized that there was an analytics and reporting forum so I am asking here in hopes that someone can steer me in the right direction.

RE: https://moodle.org/mod/forum/discuss.php?d=239747

I need to run a report to find out how long it takes between the time a student submits any sort of human-gradable assignment and the time it is graded so that we can monitor our staff's efficiency. Right now I am going for a query similar to the one below. I still need to add in the actual assignments but I am not sure which items in moodle could possibly fit into the "submitted but waiting to be graded" category.

Which tables am I missing for this query and is there anything I am obviously doing wrong here? I am relatively new to moodle but not new to programing or SQL.

SELECT c.shortname AS course_name
     , cm.id AS cmid
     , cm.course AS courseid
     , md.name AS modname
     , gi.itemname AS itemname
     , u.firstname AS student_first
     , u.lastname AS student_last
     , f.firstname AS grader_first
     , f.lastname AS grader_last
     , NULLIF(GREATEST(IFNULL(g.overridden, 0)
                      ,IFNULL(g.timecreated, 0)
                      ,IFNULL(g.timemodified, 0))
             ,0) AS graded_unixtimestamp
  
  FROM mdl_user u
  JOIN mdl_user_enrolments ue
    ON (u.id = ue.userid)
  JOIN mdl_enrol e
    ON (e.id = ue.enrolid)
  JOIN mdl_course c
    ON (c.id = e.courseid)
  JOIN mdl_course_modules cm
    ON (c.id = cm.course)
  JOIN mdl_modules md
    ON (md.id = cm.module)
  JOIN mdl_grade_items gi
    ON (
        gi.itemmodule = md.name
        AND 
        gi.iteminstance = cm.instance
        AND gi.courseid = cm.course)
  LEFT
  JOIN mdl_grade_grades g
    ON (gi.id = g.itemid AND g.userid = u.id)
  LEFT
  JOIN mdl_user f
    ON (g.usermodified = u.id)