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20 lines
761 B
Markdown
20 lines
761 B
Markdown
---
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toc_priority: 41
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toc_title: SAMPLE BY
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---
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# Manipulating Sampling-Key Expressions {#manipulations-with-sampling-key-expressions}
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Syntax:
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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).
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The command is lightweight in the sense that it only changes metadata. The primary key must contain the new sample key.
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!!! note "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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