MySQL order by 排序结果错误

2024-07-24 12:32:26

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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 |

+----+-------+------+------+

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 |

+----+-------+------+------+

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。

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