GBase 8a的注释comment 功能介绍，更新注释的方法，最大长度限制

GBase 8a数据库集群支持表的注释comment ，字段的注释，本文介绍表创建时的注释以及表注释和字段注释的装修。

建表

在建表语句里，直接在字段和表定义后面，加上comment 关键字，后面加上字符串即可。

根据现有规定，comment的最大长度是2000个字符。

gbase> create table testcomment(id int comment '编号', name varchar(100) comment '姓名') comment '测试注释';
Query OK, 0 rows affected (Elapsed: 00:00:00.29)

gbase> show create table testcomment;
+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table       | Create Table                                                                                                                                                                                                            |
+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| testcomment | CREATE TABLE "testcomment" (
  "id" int(11) DEFAULT NULL COMMENT '编号',
  "name" varchar(100) DEFAULT NULL COMMENT '姓名'
) ENGINE=EXPRESS DEFAULT CHARSET=utf8 TABLESPACE='sys_tablespace' COMMENT='测试注释'         |
+-------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)

gbase>

更改表注释

用alter table TABLENAME comment '新的注释内容’对表的注释进行修改。

gbase> alter table testcomment comment '测试表注释修改';
Query OK, 0 rows affected (Elapsed: 00:00:00.12)

gbase> show create table testcomment;
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table       | Create Table                                                                                                                                                                                                                     |
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| testcomment | CREATE TABLE "testcomment" (
  "id" int(11) DEFAULT NULL COMMENT '编号',
  "name" varchar(100) DEFAULT NULL COMMENT '姓名'
) ENGINE=EXPRESS DEFAULT CHARSET=utf8 TABLESPACE='sys_tablespace' COMMENT='测试表注释修改'            |
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)

更改列注释

用alter table TABLENAME modify 列名字 列定义 comment进行修改。

注意列的原有定义要保持，包括类型，NOT NULL， default等。

gbase> alter table testcomment  modify id int comment '测试编号修改';
Query OK, 0 rows affected (Elapsed: 00:00:00.14)
Records: 0  Duplicates: 0  Warnings: 0

gbase> show create table testcomment;
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table       | Create Table                                                                                                                                                                                                                                 |
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| testcomment | CREATE TABLE "testcomment" (
  "id" int(11) DEFAULT NULL COMMENT '测试编号修改',
  "name" varchar(100) DEFAULT NULL COMMENT '姓名'
) ENGINE=EXPRESS DEFAULT CHARSET=utf8 TABLESPACE='sys_tablespace' COMMENT='测试表注释修改'                |
+-------------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)

添加注释

添加注释和修改定时完全一样的操作。

gbase> desc t1;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | YES  | MUL | NULL    |       |
| value | int(11)  | YES  |     | NULL    |       |
| birth | datetime | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+
3 rows in set (Elapsed: 00:00:00.01)

gbase> alter table t1 comment '添加数值';
Query OK, 0 rows affected (Elapsed: 00:00:00.15)

gbase> show create table t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                           |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE "t1" (
  "id" int(11) DEFAULT NULL,
  "value" int(11) DEFAULT NULL,
  "birth" datetime DEFAULT NULL,
  KEY "idx_id" ("id") KEY_BLOCK_SIZE=4096 USING HASH GLOBAL
) ENGINE=EXPRESS DEFAULT CHARSET=utf8 TABLESPACE='sys_tablespace' COMMENT='添加数值'     |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)

gbase> alter table t1 modify id int comment '添加注释';
Query OK, 0 rows affected (Elapsed: 00:00:00.16)
Records: 0  Duplicates: 0  Warnings: 0

gbase> show create table t1;
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                  |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE "t1" (
  "id" int(11) DEFAULT NULL COMMENT '添加注释',
  "value" int(11) DEFAULT NULL,
  "birth" datetime DEFAULT NULL,
  KEY "idx_id" ("id") KEY_BLOCK_SIZE=4096 USING HASH GLOBAL
) ENGINE=EXPRESS DEFAULT CHARSET=utf8 TABLESPACE='sys_tablespace' COMMENT='添加数值'         |
+-------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (Elapsed: 00:00:00.00)