order by 排序机制

order by a,b desc 和 order by a desc,b desc 区别

order by a,b desc

select * from demo order by create_time,id desc

MySql中，order by 默认是按升序排列的。 显然，不写排序方式，默认是升序。

order by a desc,b desc

select * from demo order by create_time desc,id desc

DROP TABLE IF EXISTS `demo`;
CREATE TABLE `demo`  (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci DEFAULT NULL,
  `create_time` datetime(0) DEFAULT NULL,
  PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB AUTO_INCREMENT = 11 CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

INSERT INTO `demo` VALUES (1, '11', '2021-11-04 10:33:35');
INSERT INTO `demo` VALUES (3, '11', '2021-11-04 10:33:36');
INSERT INTO `demo` VALUES (4, '11', '2021-11-04 10:33:37');
INSERT INTO `demo` VALUES (5, '11', '2021-11-04 10:33:38');
INSERT INTO `demo` VALUES (6, '11', '2021-11-04 10:33:35');
INSERT INTO `demo` VALUES (7, '11', '2021-11-04 10:33:35');
INSERT INTO `demo` VALUES (8, '11', '2021-11-04 10:33:35');
INSERT INTO `demo` VALUES (9, '11', '2021-11-04 10:33:35');
INSERT INTO `demo` VALUES (10, '11', '2021-11-04 10:33:35');

总结

（1）在MySql中，使用 ASC 或 DESC 关键字来设置查询结果是按升序或降序排列。
（2）尤其非常特别重要：默认按升序（ASC）排列。
（3）order by 后可加2个（或多个）字段，字段之间用英文逗号隔开。
（4）若A用升序，B用降序，SQL该这样写：order by A ASC, B DESC; 默认同理，也可以这样写：order by A, B DESC;
（5）若A、B都用降序，必须用两个DESC，order by A DESC, B DESC;
（6）多个字段时，优先级按先后顺序而定。

参考

mysql order by 多个字段排序)