ClickHouse/docs/en/sql-reference/statements/alter/comment.md

slug	sidebar_position	sidebar_label
/en/sql-reference/statements/alter/comment	51	COMMENT

ALTER TABLE ... MODIFY COMMENT

Adds, modifies, or removes comment to the table, regardless if it was set before or not. Comment change is reflected in both system.tables and SHOW CREATE TABLE query.

Syntax

ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY COMMENT 'Comment'

Examples

Creating a table with comment (for more information, see the COMMENT clause):

CREATE TABLE table_with_comment
(
    `k` UInt64,
    `s` String
)
ENGINE = Memory()
COMMENT 'The temporary table';

Modifying the table comment:

ALTER TABLE table_with_comment MODIFY COMMENT 'new comment on a table';
SELECT comment FROM system.tables WHERE database = currentDatabase() AND name = 'table_with_comment';

Output of a new comment:

┌─comment────────────────┐
│ new comment on a table │
└────────────────────────┘

Removing the table comment:

ALTER TABLE table_with_comment MODIFY COMMENT '';
SELECT comment FROM system.tables WHERE database = currentDatabase() AND name = 'table_with_comment';

Output of a removed comment:

┌─comment─┐
│         │
└─────────┘

Caveats

For Replicated tables, the comment can be different on different replicas. Modifying the comment applies to a single replica.

The feature is available since version 23.9. It does not work in previous ClickHouse versions.