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> Documentation for Manipulating SAMPLE BY expression

# Manipulating Sampling-Key Expressions

The following operations are available:

<h2 id="modify">
  MODIFY
</h2>

```sql theme={null}
ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY SAMPLE BY new_expression
```

The command changes the [sampling key](/reference/engines/table-engines/mergetree-family/mergetree) of the table to `new_expression` (an expression or a tuple of expressions). The primary key must contain the new sample key.

<h2 id="remove">
  REMOVE
</h2>

```sql theme={null}
ALTER TABLE [db].name [ON CLUSTER cluster] REMOVE SAMPLE BY
```

The command removes the [sampling key](/reference/engines/table-engines/mergetree-family/mergetree) of the table.

The commands `MODIFY` and `REMOVE` are lightweight in the sense that they only change metadata or remove files.

<Note>
  It only works for tables in the [MergeTree](/reference/engines/table-engines/mergetree-family/mergetree) family (including [replicated](/reference/engines/table-engines/mergetree-family/replication) tables).
</Note>