Question

这里也提出了同样的问题：MySQL blob: how to get just a subset of the stored data。但是，提供的解决方案不起作用，SUBSTRING和MID都无法正常使用 BLOB 数据类型。我是否错过了这个查询的内容：SELECT SUBSTRING(file, 12, 48) FROM log WHERE id=8，子集从位置1开始，长度稍微为48。谢谢。

Answer 1

SUBSTRING函数似乎在VARBINARY和BLOB上正常工作，至少在MariaDB 5.6.20：

MySQL [gps]> show create table KV \G
*************************** 1. row ***************************
       Table: KV
Create Table: CREATE TABLE `KV` (
  `kv_key` varbinary(767) NOT NULL,
  `kv_value` longblob NOT NULL,
  PRIMARY KEY (`kv_key`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin
1 row in set (0.00 sec)

MySQL [gps]> select HEX(kv_key), HEX(kv_value) from KV where kv_key > x'99' limit 10;
+--------------------------------------------------------+------------------------+
| HEX(kv_key)                                            | HEX(kv_value)          |
+--------------------------------------------------------+------------------------+
| FC0696FC1A4AD423B65465FC3D7BBBAFCBDCD8FC657A8BB203FE0F |                        |
| FC114A8CF4C01BD7                                       | 01FE589B5E0200         |
| FC114A8CF4C01BD7FCC991                                 | 55414200               |
| FC114AF271B994BF                                       | 01FE589B5E0200         |
| FC114AF271B994BFFCC991                                 | 434F50442050696C6F7400 |
| FC1A4AD423B65465                                       | 01FE589B5E0200         |
| FC1A4AD423B65465FC2550FCB7D4D490FCA2E4                 |                        |
+--------------------------------------------------------+------------------------+
10 rows in set (0.00 sec)

MySQL [gps]> select HEX(SUBSTRING(kv_key, 4, 4)), HEX(kv_value) from KV where kv_key > x'99' limit 10;
+------------------------------+------------------------+
| HEX(SUBSTRING(kv_key, 4, 4)) | HEX(kv_value)          |
+------------------------------+------------------------+
| FC1A4AD4                     |                        |
| 8CF4C01B                     | 01FE589B5E0200         |
| 8CF4C01B                     | 55414200               |
| F271B994                     | 01FE589B5E0200         |
| F271B994                     | 434F50442050696C6F7400 |
| D423B654                     | 01FE589B5E0200         |
| D423B654                     |                        |
+------------------------------+------------------------+
10 rows in set (0.01 sec)

MySQL [gps]> select HEX(kv_key), HEX(SUBSTRING(kv_value, 4, 4)) from KV where kv_key > x'99' limit 10;
+--------------------------------------------------------+--------------------------------+
| HEX(kv_key)                                            | HEX(SUBSTRING(kv_value, 4, 4)) |
+--------------------------------------------------------+--------------------------------+
| FC0696FC1A4AD423B65465FC3D7BBBAFCBDCD8FC657A8BB203FE0F |                                |
| FC114A8CF4C01BD7                                       | 9B5E0200                       |
| FC114A8CF4C01BD7FCC991                                 | 00                             |
| FC114AF271B994BF                                       | 9B5E0200                       |
| FC114AF271B994BFFCC991                                 | 44205069                       |
| FC1A4AD423B65465                                       | 9B5E0200                       |
| FC1A4AD423B65465FC2550FCB7D4D490FCA2E4                 |                                |
+--------------------------------------------------------+--------------------------------+
10 rows in set (0.00 sec)

如何获得blob的一个子集

1 个答案: