gistfile1.txt

WITH reserved AS (
	SELECT
		  COUNT(sectionID) reservedCount
		, sectionID
	FROM
		enrollmentReservation
), enrolled AS (
	SELECT
          COUNT(sectionID) enrolledCount
		, sectionID
    FROM
        Enrollment
)
SELECT
    CASE 
		WHEN seatsAvailable - reserved.reservedCount - enrolled.enrolledCount < 0 
		THEN 0 
		ELSE seatsAvailable - reserved.reservedCount - enrolled.enrolledCount 
	END totalSeats
FROM
    SECTION
LEFT OUTER
	JOIN reserved
	ON SECTION.sectionID = reserved.sectionID
LEFT OUTER
	JOIN enrolled
	ON SECTION.sectionID = reserved.sectionID
WHERE
    SectionID = 33917