sql语句中自连接可以解决的问题

1、自连接可以把同一表中属于一列的数据，平移到一行中，然后进行比较或其他操作。

实例：有两张表学生信息表和分数表；

结构和数据如下：

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for zhongxin_student
-- ----------------------------
DROP TABLE IF EXISTS `zhongxin_student`;
CREATE TABLE `zhongxin_student`  (
  `id` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '姓名',
  `age` int(11) NULL DEFAULT NULL COMMENT '年龄',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of zhongxin_student
-- ----------------------------
INSERT INTO `zhongxin_student` VALUES ('1', '张三', 11);
INSERT INTO `zhongxin_student` VALUES ('2', '李四', 22);

SET FOREIGN_KEY_CHECKS = 1;

SET NAMES utf8mb4;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
-- Table structure for zhongxin_score
-- ----------------------------
DROP TABLE IF EXISTS `zhongxin_score`;
CREATE TABLE `zhongxin_score`  (
  `id` int(11) NOT NULL,
  `sid` int(11) NULL DEFAULT NULL COMMENT '学生id',
  `course_id` int(11) NULL DEFAULT NULL COMMENT '课程id',
  `score` int(11) NULL DEFAULT NULL COMMENT '分数',
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

-- ----------------------------
-- Records of zhongxin_score
-- ----------------------------
INSERT INTO `zhongxin_score` VALUES (1, 1, 1, 70);
INSERT INTO `zhongxin_score` VALUES (2, 1, 2, 80);
INSERT INTO `zhongxin_score` VALUES (3, 2, 1, 80);
INSERT INTO `zhongxin_score` VALUES (4, 2, 2, 70);

SET FOREIGN_KEY_CHECKS = 1;

数据概览：

student：

 

 

 

score：

 

 

 要求：查询课程编号1比课程编号2分数高的学生姓名

sql语句：

//查询‘1’科目成绩比‘2’科目成绩高的学生姓名
select zs.name
from zhongxin_student zs 
join (select sid,score from zhongxin_score where course_id=1)as a on a.sid=zs.id
join (select sid,score from zhongxin_score where course_id=2)as b on b.sid=zs.id
where a.score>b.score

结果：