mysql随机取数据

SELECT * FROM `content` 
WHERE id >= (SELECT floor( RAND() * ((SELECT MAX(id) FROM `content`)-(SELECT MIN(id) FROM `content`)) + (SELECT MIN(id) FROM `content`)))  
ORDER BY id LIMIT 1; 

1. 上面那种方法效率高，但是如果含有条件的随机查询目前我用不了上面的方法，我用下面的方法，效率低一点吧

   select * from show_pictures where picture_type ='lunbotu' ORDER BY RAND() LIMIT 6;