ClickHouse/docs/en/sql-reference/statements/alter/sample-by.md

---
toc_priority: 41
toc_title: SAMPLE BY
---

# Manipulating Sampling-Key Expressions {#manipulations-with-sampling-key-expressions}

Syntax:

``` sql
ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY SAMPLE BY new_expression
```

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 command is lightweight in the sense that it only changes metadata. The primary key must contain the new sample key.

!!! note "Note"
    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).