MySQL中修改列属性时造成comment属性丢失

修改字段定义的时候，要把整个字段的全部的定义都写上，举个栗子：

原字段定义：

state  varchar(50) NOT NULL DEFAULT '' COMMENT '产品资料状态'

需求：修改字段长度为100

正确的写法：

alter table t1 modify state  varchar(100) NOT NULL DEFAULT '' COMMENT '产品资料状态'

错误的写法：

alter table t1 modify state  varchar(100)            //此操作会将NOT NULL DEFAULT '' COMMENT '产品资料状态' 信息清掉，state的定义只有 varchar(100)。

这种问题会发生在mysql、tidb中，需要特别注意，Oracle中则没有这种问题。

下面是测试过程：

mysql：修改列属性时需要写上comment完成语句

17:25:52[5.7.25-log]root->192.168.30.10[mtest]> create table t1 (id int,name varchar(100));
alter table t1 modify name  varchar(100)  COMMENT 'test_comment';
show create table t1;
Query OK, 0 rows affected (0.04 sec)

17:27:30[5.7.25-log]root->192.168.30.10[mtest]> alter table t1 modify name  varchar(100)  COMMENT 'test_comment';
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

17:27:30[5.7.25-log]root->192.168.30.10[mtest]> show create table t1;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                       |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(100) DEFAULT NULL COMMENT 'test_comment'
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

--修改列属性后comment信息丢失
17:27:30[5.7.25-log]root->192.168.30.10[mtest]> alter table t1 modify name  varchar(200);   
show create table t1;
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

17:33:26[5.7.25-log]root->192.168.30.10[mtest]> show create table t1;
+-------+-----------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                |
+-------+-----------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(200) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+-------+-----------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
--正确的操作
17:33:26[5.7.25-log]root->192.168.30.10[mtest]> alter table t1 modify name  varchar(200) COMMENT 'test_comment';   
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

17:35:08[5.7.25-log]root->192.168.30.10[mtest]> show create table t1;
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                       |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(200) DEFAULT NULL COMMENT 'test_comment'
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+-------+----------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

tidb：修改列属性时需要写上comment完成语句

17:23:43[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> create table t1 (id int,name varchar(100));
alter table t1 modify name  varchar(100)  COMMENT 'test_comment';
Query OK, 0 rows affected (0.16 sec)

17:27:01[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> alter table t1 modify name  varchar(100)  COMMENT 'test_comment';
Query OK, 0 rows affected (0.17 sec)

17:27:02[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> show create table t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                           |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(100) DEFAULT NULL COMMENT 'test_comment'
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
--修改列属性后comment信息丢失
17:27:10[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> alter table t1 modify name  varchar(200);   
show create table t1;
Query OK, 0 rows affected (0.12 sec)

17:33:20[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> show create table t1;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                    |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(200) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
--正确的操作
17:33:20[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> alter table t1 modify name  varchar(200) COMMENT 'test_comment';   
show create table t1;
Query OK, 0 rows affected (0.13 sec)

17:35:12[5.7.25-TiDB-v4.0.5]root->192.168.30.10[mtest]> show create table t1;
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                           |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t1    | CREATE TABLE `t1` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(200) DEFAULT NULL COMMENT 'test_comment'
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

oracle：修改列属性不影响comment

17:25:58 SYS@orcl11g>create table t1 (id int,name varchar2(100));

Table created.

17:27:41 SYS@orcl11g>comment on column t1.name is 'test_comment'; 

Comment created.

--oracle中看到的表结构

  CREATE TABLE "SYS"."T1"
   (    "ID" NUMBER(*,0),
        "NAME" VARCHAR2(100)
   ) ;


   COMMENT ON COLUMN "SYS"."T1"."NAME" IS 'test_comment';
   
 --修改列属性后再看下表结构
 17:27:45 SYS@orcl11g>alter table t1 modify name  varchar2(200);

Table altered.
  CREATE TABLE "SYS"."T1"
   (    "ID" NUMBER(*,0),
        "NAME" VARCHAR2(200)
   ) ;


   COMMENT ON COLUMN "SYS"."T1"."NAME" IS 'test_comment';