Сделайте пожалуйста комментарии для SQL запроса,

SELECT

groups.groupname,
Semesters.semester,
employees.lastname,
employees.firstname,
(SELECT O.ocenka FROM Ocenki O WHERE O.id_employees=employees.id AND O.id_Semesters=Ocenki.id_Semesters AND O.id_Predmet=1),
(SELECT O.ocenka FROM Ocenki O WHERE O.id_employees=employees.id AND O.id_Semesters=Ocenki.id_Semesters AND O.id_Predmet=2),
(SELECT O.ocenka FROM Ocenki O WHERE O.id_employees=employees.id AND O.id_Semesters=Ocenki.id_Semesters AND O.id_Predmet=3),
(SELECT O.ocenka FROM Ocenki O WHERE O.id_employees=employees.id AND O.id_Semesters=Ocenki.id_Semesters AND O.id_Predmet=4),
(SELECT AVG(O.ocenka) FROM Ocenki O WHERE O.id_employees=employees.id AND O.id_Semesters=Ocenki.id_Semesters)

FROM Ocenki

LEFT OUTER JOIN employees ON employees.id=Ocenki.id_employees
LEFT OUTER JOIN groups ON groups.id=employees.id_groups
LEFT OUTER JOIN Semesters ON Semesters.id=Ocenki.id_Semesters
LEFT OUTER JOIN Predmet ON Predmet.id=Ocenki.id_Predmet

WHERE
(CASE WHEN {cbFIO}=-1 THEN 1=1 ELSE employees.id={cbFIO} END) AND
(CASE WHEN {cbGroups}=-1 THEN 1=1 ELSE groups.id={cbGroups} END) AND
(CASE WHEN {cbSemester}=-1 THEN 1=1 ELSE Semesters.id={cbSemester} END)

GROUP BY employees.id, Semesters.id