UPDATE	scores			
SET		passed = TRUE		
WHERE	passed IS NULL;			

UPDATE	scores			
SET		passed = FALSE		
FROM	(			
		SELECT	subject_code,
				AVG(score) AS avg
		FROM scores
		GROUP BY subject_code
		) AS sub		
		INNER JOIN scores AS sc
			ON sc.subject_code = sub.subject_code
WHERE	sc.score >= sub.avg;