MySQL order by 排序結(jié)果錯(cuò)誤

2024-07-24 12:32:26

字體：大中小

供稿：網(wǎng)友

MySQL order by 排序結(jié)果不正確

新建一張測試表：

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 沒有索引，插入測試數(shù)據(jù)：

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);

查看表數(shù)據(jù)：

([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 |

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

根據(jù)非索引字段且有重復(fù)數(shù)據(jù)的字段 a 進(jìn)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 |

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

為 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 |

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

對于一個(gè)非唯一字段，無論是否含有索引，結(jié)果集都是不確定的。如果業(yè)務(wù)邏輯對分頁或者order by結(jié)果集有比較高的嚴(yán)格要求，請記得利用唯一鍵排序。

（編輯：武林網(wǎng)）

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