场景：
一个文章表，存放有关PHP的文章，和MySQL的文章，想从这些文章类别各自提取N条记录出来。测试表语句：
CREATE TABLE article(
id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY, cate VARCHAR(20), title VARCHAR(20));INSERT INTO article(cate, title) VALUES ('php', 'php文章1'),('mysql', 'mysql文章1'),('php', 'php文章2'),('mysql', 'mysql文章2'),('php', 'php文章3'),('mysql', 'mysql文章3'),('php', 'php文章4'),('mysql', 'mysql文章4'),('php', 'php文章5'),('mysql', 'mysql文章5'),('php', 'php文章6'),('mysql', 'mysql文章6'),('php', 'php文章7'),('mysql', 'mysql文章7'),('php', 'php文章8'),('mysql', 'mysql文章8');
方案一：
SELECT a .id , a .title , a .cate FROM article AS a LEFT JOIN article AS b ON a .cate = b .cate AND a .id < b .id GROUP BY a .id , a .title , a .cate HAVING COUNT (b .id ) < 5 ORDER BY a .id DESC;
方案二：
SELECT * FROM article AS a WHERE 5 > ( SELECT COUNT ( * ) FROM article WHERE cate = a .cate AND id > a .id ) ORDER BY a .id DESC;
