MySQL order by 排序结果不正确

新建一张测试表：

CREATE TABLE `tb1` (
 
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
 
  `a` decimal(19,2) NOT NULL,
 
  `acid` bigint(20) NOT NULL,
 
  `prid` bigint(20) NOT NULL,
 
  PRIMARY KEY (`id`),
 
  KEY `idx_prid` (`prid`),
 
  KEY `idx_acid` (`acid`)
 
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

字段 a 没有索引，插入测试数据：

INSERT INTO `tb1` (`id`, `a`, `acid`, `prid`) 
VALUES (1,2.00,3,2),(2,3.00,3,2),(3,4.00,2,3),(4,5.00,2,3),(5,6.00,2,3),(6,8.00,2,3),(7,10.00,2,3),(8,12.00,2,3),(9,16.00,2,3),(10,20.00,2,3),(11,6.00,2,4),(12,8.00,2,4),(13,10.00,2,4),(14,12.00,2,4),(15,5.00,2,2),(16,6.00,2,2);

查看表数据：

([yoon]> select * from tb1;
+----+-------+------+------+
| id | a     | acid | prid |
+----+-------+------+------+
|  1 |  2.00 |    3 |    2 |
|  2 |  3.00 |    3 |    2 |
|  3 |  4.00 |    2 |    3 |
|  4 |  5.00 |    2 |    3 |
|  5 |  6.00 |    2 |    3 |
|  6 |  8.00 |    2 |    3 |
|  7 | 10.00 |    2 |    3 |
|  8 | 12.00 |    2 |    3 |
|  9 | 16.00 |    2 |    3 |
| 10 | 20.00 |    2 |    3 |
| 11 |  6.00 |    2 |    4 |
| 12 |  8.00 |    2 |    4 |
| 13 | 10.00 |    2 |    4 |
| 14 | 12.00 |    2 |    4 |
| 15 |  5.00 |    2 |    2 |
| 16 |  6.00 |    2 |    2 |
+----+-------+------+------+

根据非索引字段且有重复数据的字段 a 进行 order by 排序：

([yoon]> select * from tb1 order by a desc ;
+----+-------+------+------+
| id | a     | acid | prid |
+----+-------+------+------+
| 10 | 20.00 |    2 |    3 |
|  9 | 16.00 |    2 |    3 |
| 14 | 12.00 |    2 |    4 |
|  8 | 12.00 |    2 |    3 |
| 13 | 10.00 |    2 |    4 |
|  7 | 10.00 |    2 |    3 |
| 12 |  8.00 |    2 |    4 |
|  6 |  8.00 |    2 |    3 |
| 11 |  6.00 |    2 |    4 |
| 16 |  6.00 |    2 |    2 |
|  5 |  6.00 |    2 |    3 |
|  4 |  5.00 |    2 |    3 |
| 15 |  5.00 |    2 |    2 |
|  3 |  4.00 |    2 |    3 |
|  2 |  3.00 |    3 |    2 |
|  1 |  2.00 |    3 |    2 |
+----+-------+------+------+

order by 和 limit 一起使用：

([yoon]> select * from tb1 order by a desc limit 4;
+----+-------+------+------+
| id | a     | acid | prid |
+----+-------+------+------+
| 10 | 20.00 |    2 |    3 |
|  9 | 16.00 |    2 |    3 |
| 14 | 12.00 |    2 |    4 |
|  8 | 12.00 |    2 |    3 |
+----+-------+------+------+

为 a 字段添加索引：

([yoon]> alter table tb1 add index idx_a(a);
Query OK, 0 rows affected (0.05 sec)

([yoon]> select * from tb1 order by a desc limit 4;
+----+-------+------+------+
| id | a     | acid | prid |
+----+-------+------+------+
| 10 | 20.00 |    2 |    3 |
|  9 | 16.00 |    2 |    3 |
| 14 | 12.00 |    2 |    4 |
|  8 | 12.00 |    2 |    3 |
+----+-------+------+------+

排序的时候再添加一个字段 id ：

([yoon]> select * from tb1 order by a desc,id desc limit 4;
+----+-------+------+------+
| id | a     | acid | prid |
+----+-------+------+------+
| 10 | 20.00 |    2 |    3 |
|  9 | 16.00 |    2 |    3 |
| 14 | 12.00 |    2 |    4 |
|  8 | 12.00 |    2 |    3 |
+----+-------+------+------+

对于一个非唯一字段，无论是否含有索引，结果集都是不确定的。如果业务逻辑对分页或者order by结果集有比较高的严格要求 ，请记得利用唯一键排序。