عبارات ALTER DATABASE ... MODIFY COMMENT

CREATE DATABASE database_with_comment ENGINE = Memory COMMENT 'The temporary database';

ALTER DATABASE database_with_comment 
MODIFY COMMENT 'new comment on a database';

SELECT comment 
FROM system.databases 
WHERE name = 'database_with_comment';

┌─comment─────────────────┐
│ new comment on database │
└─────────────────────────┘

ALTER DATABASE database_with_comment 
MODIFY COMMENT '';

SELECT comment 
FROM system.databases 
WHERE  name = 'database_with_comment';

┌─comment─┐
│         │
└─────────┘