problems  CREATE TABLE `problems` (                                                  
            `problem_id` bigint(11) NOT NULL auto_increment COMMENT '问题表 ID',     
            `problem_type` int(1) NOT NULL COMMENT '问题类型 1.理论题,2实践题',      
            `problem_title` varchar(200) NOT NULL COMMENT '问题',                    
            `problem_option_a` varchar(100) default NULL COMMENT 'A 选项',           
            `problem_option_b` varchar(100) default NULL COMMENT 'B 选项',           
            `problem_option_c` varchar(100) default NULL COMMENT 'C 选项',           
            `problem_option_d` varchar(100) default NULL COMMENT 'D 选项',           
            `correct_answer` int(1) default '0' COMMENT '正确选项 1.A,2.B,3.C,4.D',  
            `problem_is_main` int(1) default '1' COMMENT '是否主要問題 1不是 2 是',      
            `problem_image` varchar(200) default NULL COMMENT '问题图片路径',        
            `problem_remark` text COMMENT '描述',                                    
            PRIMARY KEY  (`problem_id`)                                              
          ) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=gbk   

select p.* FROM problems p,
(
 SELECT DISTINCT problem_id
   FROM
(
 SELECT problem_id,
        1 AS ord
   FROM (SELECT problem_id
           FROM problems
          WHERE problem_type = 1
            AND problem_is_main = 1
          ORDER BY rand()
          LIMIT 2) x1
  UNION ALL
 SELECT problem_id,
        1 AS ord
   FROM (SELECT problem_id
           FROM problems
          WHERE problem_type = 2
            AND problem_is_main = 1
          ORDER BY rand()
          LIMIT 2) x2
  UNION ALL
 SELECT problem_id,
        1 AS ord
   FROM (SELECT problem_id
           FROM problems
          WHERE problem_type = 3
            AND problem_is_main = 1
          ORDER BY rand()
          LIMIT 2) x3
  UNION ALL
 SELECT problem_id,
        ord
   FROM (SELECT problem_id,
                rand() + 2 AS ord
           FROM problems
          ORDER BY ord
          LIMIT 30) x4
  ORDER BY ord
) x
  ORDER BY ord
  LIMIT 30
) sub
where p.problem_id = sub.problem_id