标签：acid set values efault insert alt query ODB 业务逻辑
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。
标签：acid set values efault insert alt query ODB 业务逻辑
原文地址：https://www.cnblogs.com/hankyoon/p/12614581.html
