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34 lines
1.1 KiB
Markdown
34 lines
1.1 KiB
Markdown
---
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slug: /en/sql-reference/statements/alter/sample-by
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sidebar_position: 41
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sidebar_label: SAMPLE BY
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title: "Manipulating Sampling-Key Expressions"
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---
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# Manipulating SAMPLE BY expression
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The following operations are available:
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## MODIFY
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``` sql
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ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY SAMPLE BY new_expression
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```
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The command changes the [sampling key](../../../engines/table-engines/mergetree-family/mergetree.md) of the table to `new_expression` (an expression or a tuple of expressions). The primary key must contain the new sample key.
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## REMOVE
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``` sql
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ALTER TABLE [db].name [ON CLUSTER cluster] REMOVE SAMPLE BY
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```
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The command removes the [sampling key](../../../engines/table-engines/mergetree-family/mergetree.md) of the table.
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The commands `MODIFY` and `REMOVE` are lightweight in the sense that they only change metadata or remove files.
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:::note
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It only works for tables in the [MergeTree](../../../engines/table-engines/mergetree-family/mergetree.md) family (including [replicated](../../../engines/table-engines/mergetree-family/replication.md) tables).
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:::
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