MySQL order by 排序結果不正確

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，插入測試資料：

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);

檢視表資料：

([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 |
+----+-------+------+------+

根據非索引欄位且有重複資料的字段 a 進行 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 |
+----+-------+------+------+

為 a 字段新增索引：

([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 |
+----+-------+------+------+

排序的時候再新增乙個字段 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 |
+----+-------+------+------+

對於乙個非唯一字段，無論是否含有索引，結果集都是不確定的。如果業務邏輯對分頁或者order by結果集有比較高的嚴格要求 ，請記得利用唯一鍵排序。

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