mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-16 20:53:27 +00:00
66 lines
1.7 KiB
Markdown
66 lines
1.7 KiB
Markdown
---
|
|
slug: /en/sql-reference/statements/alter/comment
|
|
sidebar_position: 51
|
|
sidebar_label: 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](../../../operations/system-tables/tables.md) and `SHOW CREATE TABLE` query.
|
|
|
|
**Syntax**
|
|
|
|
``` sql
|
|
ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY COMMENT 'Comment'
|
|
```
|
|
|
|
**Examples**
|
|
|
|
Creating a table with comment (for more information, see the [COMMENT](../../../sql-reference/statements/create/table.md#comment-table) clause):
|
|
|
|
``` sql
|
|
CREATE TABLE table_with_comment
|
|
(
|
|
`k` UInt64,
|
|
`s` String
|
|
)
|
|
ENGINE = Memory()
|
|
COMMENT 'The temporary table';
|
|
```
|
|
|
|
Modifying the table comment:
|
|
|
|
``` sql
|
|
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:
|
|
|
|
```text
|
|
┌─comment────────────────┐
|
|
│ new comment on a table │
|
|
└────────────────────────┘
|
|
```
|
|
|
|
Removing the table comment:
|
|
|
|
``` sql
|
|
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:
|
|
|
|
```text
|
|
┌─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.
|