select    primary_count as primaryCount,      primary_score as primaryScore,      junior_count as juniorCount,        junior_score as juniorScore,        senior_count as seniorCount,        senoir_score as senoirScore,        total_score as totalScore,      pass_score as passScore from pd_paper pwhere p.is_valid = ‘1‘order by RAND() limit 1 分析：ORDER BY从句里面不能使用RAND()函数，因为这样会导致数据列被多次扫描。测试发现这样效率非常低。一个15万余条的库，查询5条数据，要8秒以上。 You cannot use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. 更高效的做法：查询max(id) * rand()来随机获取数据。  SELECT * FROM `table` AS t1 JOIN (    SELECT ROUND(RAND()     * (SELECT MAX(id) FROM `table`)) AS id) AS t2 WHERE t1.id >= t2.id ORDER BY t1.id ASC LIMIT 5; 但是这样会产生连续的5条记录。解决办法只能是每次查询一条，查询5次。即便如此也值得，因为15万条的表，查询只需要0.01秒不到。采用join 语法  可以实现真正的随机。 SELECT * FROM `table` WHERE id >= (SELECT FLOOR( MAX(id) * RAND()) FROM `table` ) ORDER BY id LIMIT 1; 把语句完善一下，加上MIN(id)的判断。如果没有加上MIN(id)的判断，结果有一半的时间总是查询到表中的前面几行。完整的语句： ①  where 子句 SELECT * FROM `table` WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `table`)- (SELECT MIN(id) FROM `table`)) + (SELECT MIN(id) FROM `table`)))  ORDER BY id LIMIT 1; ② join SELECT * FROM `table` AS t1 JOIN (    SELECT ROUND(RAND()     * ((SELECT MAX(id) FROM `table`)-(SELECT MIN(id) FROM `table`))    +(SELECT MIN(id) FROM `table`)) AS id) AS t2 WHERE t1.id >= t2.id ORDER BY t1.id LIMIT 1; #随机查询一套考卷定义SELECT    p.primary_count as primaryCount,        p.primary_score as primaryScore,        p.junior_count as juniorCount,      p.junior_score as juniorScore,      p.senior_count as seniorCount,      p.senoir_score as senoirScore,      p.total_score   as totalScore,      p.pass_score as passScore    FROM     pd_paper AS pJOIN (    SELECT ROUND(        RAND()         *((SELECT MAX(id) FROM pd_paper)-(SELECT MIN(id) FROM pd_paper))        +(SELECT MIN(id) FROM pd_paper)    ) AS id)     AS p2 WHERE p.id >= p2.id ORDER BY p.id LIMIT 1;  最后在程序对这两个语句进行分别查询10次，前者花费时间 0.147433 秒后者花费时间 0.015130 秒看来采用JOIN的语法比直接在WHERE中使用函数效率还要高很多。 附加一个复杂sql：按题目类型（三种）随机查询全部考题信息 SELECT * from (    select        p.id as id,        p.title as title,           p.question as question,         p.answer as answer,        p.crt_time as crtTime       from         pd_problem p    join        (SELECT ROUND(RAND()             * ((SELECT MAX(pp.id) FROM pd_problem pp)-(SELECT MIN(pp.id) FROM pd_problem pp))            +(SELECT MIN(pp.id) FROM pd_problem pp)) AS pid) AS p2     where p.id >= p2.pid    and p.is_valid = ‘1‘    and p.paper_type = ‘1‘    and p.paper_class = ‘0‘    order by p.id limit 5 ) as t1 union all  SELECT * from (    select        p.id as id,        p.title as title,           p.question as question,         p.answer as answer,        p.crt_time as crtTime       from         pd_problem p    join        (SELECT ROUND(RAND()             * ((SELECT MAX(pp.id) FROM pd_problem pp)-(SELECT MIN(pp.id) FROM pd_problem pp))            +(SELECT MIN(pp.id) FROM pd_problem pp)) AS pid) AS p2     where p.id >= p2.pid    and p.is_valid = ‘1‘    and p.paper_type = ‘1‘    and p.paper_class = ‘1‘    order by p.id limit 5 ) as t2 union all SELECT * from (    select        p.id as id,        p.title as title,           p.question as question,         p.answer as answer,        p.crt_time as crtTime       from         pd_problem p    join        (SELECT ROUND(RAND()             * ((SELECT MAX(pp.id) FROM pd_problem pp)-(SELECT MIN(pp.id) FROM pd_problem pp))            +(SELECT MIN(pp.id) FROM pd_problem pp)) AS pid) AS p2     where p.id >= p2.pid    and p.is_valid = ‘1‘    and p.paper_type = ‘1‘    and p.paper_class = ‘2‘    order by p.id limit 5 ) as t3