oracle 选择最频繁出现之前，５文章数据

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

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