查询-mysql无限极分类排序问题

表是这样设计的

2，4,5字段请无视
现在要查询出来这样格式的数据
一级分类
二级分类A
三级分类A
三级分类B
二级分类B
三级分类A
三级分类B
我使用的sql为 select * from vn_erp_healthprice order by sorts

sorts字段为varchar类型
查询的结果如下

排序结果满足业务要求,但为什么1-11排在1-2前面
还有 根据字符串排序这是个什么原理来着 好久没用过这种分类了 一时想不起来了
求大神赐教

表sql如下

 /*Navicat MySQL Data TransferSource Server         : localhostSource Server Version : 50621Source Host           : 127.0.0.1:3306Source Database       : testTarget Server Type    : MYSQLTarget Server Version : 50621File Encoding         : 65001Date: 2015-07-15 16:38:49*/SET FOREIGN_KEY_CHECKS=0;-- ------------------------------ Table structure for `vn_erp_healthprice`-- ----------------------------DROP TABLE IF EXISTS `vn_erp_healthprice`;CREATE TABLE `vn_erp_healthprice` (  `id` int(10) NOT NULL AUTO_INCREMENT COMMENT '主键',  `lyid` varchar(10) NOT NULL DEFAULT '' COMMENT '维修项id',  `name` varchar(100) NOT NULL DEFAULT '' COMMENT '维修项名称',  `price` varchar(10) NOT NULL DEFAULT '' COMMENT '工时单价',  `hours` varchar(10) NOT NULL DEFAULT '' COMMENT '工时',  `parent_id` int(11) NOT NULL DEFAULT '0',  `sorts` varchar(20) NOT NULL DEFAULT '' COMMENT '排序',  PRIMARY KEY (`id`),  KEY `hid` (`lyid`)) ENGINE=InnoDB AUTO_INCREMENT=27 DEFAULT CHARSET=utf8;-- ------------------------------ Records of vn_erp_healthprice-- ----------------------------INSERT INTO `vn_erp_healthprice` VALUES ('1', '', '保养(一级分类)', '', '', '0', '1');INSERT INTO `vn_erp_healthprice` VALUES ('2', '', '更换(二级分类)', '', '', '1', '1-2');INSERT INTO `vn_erp_healthprice` VALUES ('3', '1', '更换机油', '', '', '2', '1-2-3');INSERT INTO `vn_erp_healthprice` VALUES ('4', '2', '更换机油滤清器', '', '', '2', '1-2-4');INSERT INTO `vn_erp_healthprice` VALUES ('5', '3', '更换空气滤清器', '', '', '2', '1-2-5');INSERT INTO `vn_erp_healthprice` VALUES ('6', '4', '更换燃油滤清器', '', '', '2', '1-2-6');INSERT INTO `vn_erp_healthprice` VALUES ('7', '5', '更换全部火花塞', '', '', '2', '1-2-7');INSERT INTO `vn_erp_healthprice` VALUES ('8', '6', '检查助力转向油', '', '', '11', '1-2-8');INSERT INTO `vn_erp_healthprice` VALUES ('9', '7', '更换整车制动液', '', '', '2', '1-2-9');INSERT INTO `vn_erp_healthprice` VALUES ('10', '8', '检查自动变速箱油', '', '', '11', '1-11-10');INSERT INTO `vn_erp_healthprice` VALUES ('11', '', '检查(二级分类)', '', '', '1', '1-11');INSERT INTO `vn_erp_healthprice` VALUES ('12', '9', '检查或者更换手动变速箱油', '', '', '11', '1-11-12');INSERT INTO `vn_erp_healthprice` VALUES ('13', '10', '检查发动机正时皮带', '', '', '11', '1-11-13');INSERT INTO `vn_erp_healthprice` VALUES ('14', '11', '更换空调滤清器', '', '', '2', '1-2-14');INSERT INTO `vn_erp_healthprice` VALUES ('15', '', '定期(二级分类)', '', '', '1', '1-15');INSERT INTO `vn_erp_healthprice` VALUES ('16', '12', '定期添加冷媒', '', '', '15', '1-15-16');INSERT INTO `vn_erp_healthprice` VALUES ('17', '13', '检查空调管路', '', '', '11', '1-11-17');INSERT INTO `vn_erp_healthprice` VALUES ('18', '14', '更换防冻冷却液', '', '', '2', '1-2-18');INSERT INTO `vn_erp_healthprice` VALUES ('19', '15', '检查厚度、调整，必要时更换前刹车片', '', '', '11', '1-11-19');INSERT INTO `vn_erp_healthprice` VALUES ('20', '16', '检查厚度、调整，必要时更换前刹车盘', '', '', '11', '1-11-20');INSERT INTO `vn_erp_healthprice` VALUES ('21', '17', '检查厚度、调整，必要时更换后刹车片（蹄）', '', '', '11', '1-11-21');INSERT INTO `vn_erp_healthprice` VALUES ('22', '18', '检查厚度、调整，必要时更换后刹车盘（鼓）', '', '', '11', '1-11-21');INSERT INTO `vn_erp_healthprice` VALUES ('23', '', '调整(二级分类)', '', '', '1', '1-22');INSERT INTO `vn_erp_healthprice` VALUES ('24', '19', '调整雨刮（雨刷）', '', '', '23', '1-22-23');INSERT INTO `vn_erp_healthprice` VALUES ('25', '20', '检查电瓶', '', '', '11', '1-11-24');INSERT INTO `vn_erp_healthprice` VALUES ('26', '21', '检查减震器', '', '', '11', '1-11-25');