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"　　这不能写，只是为了看的清楚