thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-11-10 01:25:21 +00:00
Code Issues Packages Projects Releases Wiki Activity

Fix docs

Browse Source
This commit is contained in:
János Benjamin Antal 2024-02-01 14:50:29 +00:00
parent d322accd89
commit a0935770bb
1 changed files with 13 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
docs/en/sql-reference/statements/alter/column.md
View File

@ -139,8 +139,8 @@ ALTER TABLE visits COMMENT COLUMN browser 'This column shows the browser used fo
## MODIFY COLUMN
``` sql
MODIFY COLUMN [IF EXISTS] name [type] [default_expr] [codec] [TTL] [AFTER name_after | FIRST]
ALTER COLUMN [IF EXISTS] name TYPE [type] [default_expr] [codec] [TTL] [AFTER name_after | FIRST]
MODIFY COLUMN [IF EXISTS] name [type] [default_expr] [codec] [TTL] [settings] [AFTER name_after | FIRST]
ALTER COLUMN [IF EXISTS] name TYPE [type] [default_expr] [codec] [TTL] [settings] [AFTER name_after | FIRST]
```
This query changes the `name` column properties:
@ -153,10 +153,14 @@ This query changes the `name` column properties:
- TTL
- Column-level Settings
For examples of columns compression CODECS modifying, see [Column Compression Codecs](../create/table.md/#codecs).
For examples of columns TTL modifying, see [Column TTL](/docs/en/engines/table-engines/mergetree-family/mergetree.md/#mergetree-column-ttl).
For examples of colum-level settings modifying, see [Column-level Settings](/docs/en/engines/table-engines/mergetree-family/mergetree.md/#column-level-settings).
If the `IF EXISTS` clause is specified, the query won’t return an error if the column does not exist.
When changing the type, values are converted as if the [toType](/docs/en/sql-reference/functions/type-conversion-functions.md) functions were applied to them. If only the default expression is changed, the query does not do anything complex, and is completed almost instantly.
@ -209,7 +213,7 @@ The `ALTER` query for changing columns is replicated. The instructions are saved
## MODIFY COLUMN REMOVE
Removes one of the column properties: `DEFAULT`, `ALIAS`, `MATERIALIZED`, `CODEC`, `COMMENT`, `TTL`, `SETTING`.
Removes one of the column properties: `DEFAULT`, `ALIAS`, `MATERIALIZED`, `CODEC`, `COMMENT`, `TTL`, `SETTINGS`.
Syntax:
@ -237,7 +241,7 @@ Modify a column setting.
Syntax:
```sql
ALTER TABLE table_name MODIFY COLUMN MODIFY SETTING name=value,...;
ALTER TABLE table_name MODIFY COLUMN column_name MODIFY SETTING name=value,...;
```
**Example**
@ -245,7 +249,7 @@ ALTER TABLE table_name MODIFY COLUMN MODIFY SETTING name=value,...;
Modify column's `max_compress_block_size` to `1MB`:
```sql
ALTER TABLE table_name MODIFY COLUMN MODIFY SETTING max_compress_block_size = 1048576;
ALTER TABLE table_name MODIFY COLUMN column_name MODIFY SETTING max_compress_block_size = 1048576;
```
## MODIFY COLUMN RESET SETTING
@ -255,21 +259,21 @@ Reset a column setting, also removes the setting declaration in the column expre
Syntax:
```sql
ALTER TABLE table_name MODIFY COLUMN RESET SETTING name,...;
ALTER TABLE table_name MODIFY COLUMN column_name RESET SETTING name,...;
```
**Example**
Remove column setting `max_compress_block_size` to `1MB`:
Reset column setting `max_compress_block_size` to it's default value:
```sql
ALTER TABLE table_name MODIFY COLUMN REMOVE SETTING max_compress_block_size;
ALTER TABLE table_name MODIFY COLUMN column_name RESET SETTING max_compress_block_size;
```
## MATERIALIZE COLUMN
Materializes or updates a column with an expression for a default value (`DEFAULT` or `MATERIALIZED`).
It is used if it is necessary to add or update a column with a complicated expression, because evaluating such an expression directly on `SELECT` executing turns out to be expensive.
It is used if it is necessary to add or update a column with a complicated expression, because evaluating such an expression directly on `SELECT` executing turns out to be expensive.
Implemented as a [mutation](/docs/en/sql-reference/statements/alter/index.md#mutations).
Syntax: