Greatest n per group with multiple criteria for greatest - postgresql

Question

I need to select the largest, most recent or currently active term across a number of schools, with the assumption that is possible for a school to have multiple concurrent terms (ie, one term that honors students are registered in, and another for non honors). Also need to take into account the end date, as the honors term may have the same start date but may be year long instead of just a semester, and I want the semester.
Code looks something like this:
SELECT t.school_id, t.term_id, COUNT(s.id) AS size, t.start_date, t.end_date
FROM term t
INNER JOIN students s ON t.term_id = s.term_id
WHERE t.school_id = (some school id)
GROUP BY t.school_id, t.term_id
ORDER BY t.start_date DESC, t.end_date ASC, size DESC LIMIT 1;
This works perfectly to find the largest currently or most recently active term, but I want to be able to eliminate the WHERE t.school_id = (some school id) part.
A standard greatest n per group can easily choose the largest OR most recent term, but I need to select the most recent term that ends soonest with the largest number of students.

Answer 1

Not sure I am interpreting your question correctly. Would be easier if you had supplied table definitions including primary and foreign keys.
If you want the the most recent term that ends soonest with the largest number of students per school, this might do it:
SELECT DISTINCT ON (t.school_id)
t.school_id, t.term_id, s.size, t.start_date, t.end_date
FROM term t
JOIN (
SELECT term_id, COUNT(s.id) AS size
FROM students
GROUP BY term_id
) s USING (term_id)
ORDER BY t.school_id, t.start_date DESC, t.end_date, size DESC;
More explanation for DISTINCT ON in this related answer:
Select first row in each GROUP BY group?