COMMENT¶
为现有对象添加注释或覆盖现有注释。
可以将注释添加到所有对象（用户、角色、仓库、数据库、表等）。您可使用此命令为各个表列添加注释，但不能为列上的约束添加注释。
语法¶
COMMENT [IF EXISTS] ON <object_type> <object_name> IS '<string_literal>';
COMMENT [IF EXISTS] ON COLUMN <table_name>.<column_name> IS '<string_literal>';
使用说明¶
除此命令之外，您还可以在创建或更改对象时添加注释：
在 CREATE <object> 或 ALTER * <object>* 命令中指定
COMMENT参数。COMMENT参数还可以在 ALTER * <object>* 命令中使用，以便随时更改现有注释。
向表列中添加注释时使用的语法略有不同：
可以在创建时添加注释，方法是在列声明后面加上
COMMENT关键字（而不是属性）。然后可以使用此命令更改注释。
若要向约束添加注释，请使用 CREATE TABLE 或 CREATE | ALTER TABLE ...CONSTRAINT 命令。
DESCRIBE TABLE 输出不显示表约束的注释，例如多列主键。要查看这些注释，请查询 TABLE_CONSTRAINTS 视图。
关于元数据：
注意
客户应确保在使用 Snowflake 服务时，不会将个人数据（用户对象除外）、敏感数据、出口管制数据或其他受监管数据作为元数据输入。有关更多信息，请参阅 Snowflake 中的元数据字段。
示例¶
首先创建一个带有注释的架构，然后覆盖该注释：
CREATE SCHEMA my_schema COMMENT='this is comment1';
SHOW SCHEMAS LIKE '%schema%';
+-------------------------------+--------------------+------------+------------+---------------+--------------+-----------------------------------------------------------+---------+----------------+
| created_on | name | is_default | is_current | database_name | owner | comment | options | retention_time |
|-------------------------------+--------------------+------------+------------+---------------+--------------+-----------------------------------------------------------+---------+----------------|
| 2018-06-26 11:06:55.994 -0700 | INFORMATION_SCHEMA | N | N | YDB | | Views describing the contents of schemas in this database | | 1 |
| 2018-06-26 11:06:30.532 -0700 | MY_SCHEMA | N | Y | YDB | SYSADMIN | this is comment1 | | 1 |
+-------------------------------+--------------------+------------+------------+---------------+--------------+-----------------------------------------------------------+---------+----------------+
COMMENT ON SCHEMA my_schema IS 'now comment2';
SHOW SCHEMAS LIKE '%schema%';
+-------------------------------+--------------------+------------+------------+---------------+--------------+-----------------------------------------------------------+---------+----------------+
| created_on | name | is_default | is_current | database_name | owner | comment | options | retention_time |
|-------------------------------+--------------------+------------+------------+---------------+--------------+-----------------------------------------------------------+---------+----------------|
| 2018-06-26 11:06:55.994 -0700 | INFORMATION_SCHEMA | N | N | YDB | | Views describing the contents of schemas in this database | | 1 |
| 2018-06-26 11:06:30.532 -0700 | MY_SCHEMA | N | Y | YDB | SYSADMIN | now comment2 | | 1 |
+-------------------------------+--------------------+------------+------------+---------------+--------------+-----------------------------------------------------------+---------+----------------+
创建一个带有表列注释的表，然后覆盖该注释：
CREATE TABLE my_table(my_column string COMMENT 'this is comment3');
DESC TABLE my_table;
+-----------+-------------------+--------+-------+---------+-------------+------------+-------+------------+------------------+
| name | type | kind | null? | default | primary key | unique key | check | expression | comment |
|-----------+-------------------+--------+-------+---------+-------------+------------+-------+------------+------------------|
| MY_COLUMN | VARCHAR(16777216) | COLUMN | Y | NULL | N | N | NULL | NULL | this is comment3 |
+-----------+-------------------+--------+-------+---------+-------------+------------+-------+------------+------------------+
COMMENT ON COLUMN my_table.my_column IS 'now comment4';
DESC TABLE my_table;
+-----------+-------------------+--------+-------+---------+-------------+------------+-------+------------+--------------+
| name | type | kind | null? | default | primary key | unique key | check | expression | comment |
|-----------+-------------------+--------+-------+---------+-------------+------------+-------+------------+--------------|
| MY_COLUMN | VARCHAR(16777216) | COLUMN | Y | NULL | N | N | NULL | NULL | now comment4 |
+-----------+-------------------+--------+-------+---------+-------------+------------+-------+------------+--------------+
创建带有注释的视图，然后覆盖该注释：
CREATE VIEW my_view comment='this is comment5' AS (SELECT * FROM my_table);
SHOW VIEWS LIKE 'my_view';
+-------------------------------+---------+----------+---------------+-------------+----------+------------------+-----------------------------------------------------------------------------+-----------+
| created_on | name | reserved | database_name | schema_name | owner | comment | text | is_secure |
|-------------------------------+---------+----------+---------------+-------------+----------+------------------+-----------------------------------------------------------------------------+-----------|
| 2018-06-26 11:11:01.048 -0700 | MY_VIEW | | YDB | MY_SCHEMA | SYSADMIN | this is comment5 | CREATE VIEW my_view comment='this is comment5' AS (SELECT * FROM my_table); | false |
+-------------------------------+---------+----------+---------------+-------------+----------+------------------+-----------------------------------------------------------------------------+-----------+
COMMENT ON VIEW my_view IS 'now comment6';
SHOW VIEWS LIKE 'my_view';
+-------------------------------+---------+----------+---------------+-------------+----------+--------------+-----------------------------------------------------------------------------+-----------+
| created_on | name | reserved | database_name | schema_name | owner | comment | text | is_secure |
|-------------------------------+---------+----------+---------------+-------------+----------+--------------+-----------------------------------------------------------------------------+-----------|
| 2018-06-26 11:11:01.048 -0700 | MY_VIEW | | YDB | MY_SCHEMA | SYSADMIN | now comment6 | CREATE VIEW my_view comment='this is comment5' AS (SELECT * FROM my_table); | false |
+-------------------------------+---------+----------+---------------+-------------+----------+--------------+-----------------------------------------------------------------------------+-----------+