order by rand() 效率问题变换查询yii下使用例子

随机取n条数据，使用order by rand() ，数据量不大情况下还OK，表里数据量多就不行了。变换查询方式：

sql语句变换:

SELECT * FROM 表名 WHERE id >= ((SELECT MAX(id) FROM 表名)-(SELECT MIN(id) FROM 表名)) * RAND() + (SELECT MIN(id) FROM 表名) limit 5;

Yii下查询带 where条件使用方法

$query = new \yii\db\Query();

$query->select(‘linkname,question,linkageid,content’);

$query->from(‘question as t1’);

$query->join(“JOIN”,”(SELECT ROUND(RAND() * (SELECT MAX(qid) FROM `question`)) AS id) as t2″);

$query->where(“t1.qid >= t2.id”);

$query->andWhere([‘in’,’linkageid’,$catids]);

//$query->andWhere(“linkageid !=0”);

$query->orderBy(“t1.qid ASC”);

$query->limit($num);

$question_info = $query->all();