SELECT
	*
FROM
	(
		SELECT
			PROJECT_LISTING.MATERIAL,
			COUNT (*) AS "出现次数"
		FROM
			PROJECT_LISTING
		WHERE
			PROJECT_LISTING.MATERIAL IS NOT NULL
		GROUP BY
			PROJECT_LISTING.MATERIAL
		ORDER BY
			COUNT (*) DESC
	) WHERE　ROWNUM <= 5

	ORDER BY
			COUNT (*) DESC　是关键排序

COUNT (*) AS "出现次数"　　这个可以不写，只是为了看的明白