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MySQL order by 排序结果不正确
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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。
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