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74 lines
2.6 KiB
Markdown
74 lines
2.6 KiB
Markdown
---
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toc_title: LIMIT BY
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---
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# LIMIT BY Clause {#limit-by-clause}
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A query with the `LIMIT n BY expressions` clause selects the first `n` rows for each distinct value of `expressions`. The key for `LIMIT BY` can contain any number of [expressions](../../../sql-reference/syntax.md#syntax-expressions).
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ClickHouse supports the following syntax variants:
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- `LIMIT [offset_value, ]n BY expressions`
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- `LIMIT n OFFSET offset_value BY expressions`
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During query processing, ClickHouse selects data ordered by sorting key. The sorting key is set explicitly using an [ORDER BY](../../../sql-reference/statements/select/order-by.md) clause or implicitly as a property of the table engine. Then ClickHouse applies `LIMIT n BY expressions` and returns the first `n` rows for each distinct combination of `expressions`. If `OFFSET` is specified, then for each data block that belongs to a distinct combination of `expressions`, ClickHouse skips `offset_value` number of rows from the beginning of the block and returns a maximum of `n` rows as a result. If `offset_value` is bigger than the number of rows in the data block, ClickHouse returns zero rows from the block.
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!!! note "Note"
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`LIMIT BY` is not related to [LIMIT](../../../sql-reference/statements/select/limit.md). They can both be used in the same query.
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If you want to use column numbers instead of column names in the `LIMIT BY` clause, enable the setting [enable_positional_arguments](../../../operations/settings/settings.md#enable-positional-arguments).
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## Examples {#examples}
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Sample table:
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``` sql
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CREATE TABLE limit_by(id Int, val Int) ENGINE = Memory;
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INSERT INTO limit_by VALUES (1, 10), (1, 11), (1, 12), (2, 20), (2, 21);
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```
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Queries:
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``` sql
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SELECT * FROM limit_by ORDER BY id, val LIMIT 2 BY id
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```
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``` text
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┌─id─┬─val─┐
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│ 1 │ 10 │
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│ 1 │ 11 │
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│ 2 │ 20 │
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│ 2 │ 21 │
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└────┴─────┘
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```
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``` sql
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SELECT * FROM limit_by ORDER BY id, val LIMIT 1, 2 BY id
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```
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``` text
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┌─id─┬─val─┐
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│ 1 │ 11 │
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│ 1 │ 12 │
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│ 2 │ 21 │
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└────┴─────┘
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```
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The `SELECT * FROM limit_by ORDER BY id, val LIMIT 2 OFFSET 1 BY id` query returns the same result.
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The following query returns the top 5 referrers for each `domain, device_type` pair with a maximum of 100 rows in total (`LIMIT n BY + LIMIT`).
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``` sql
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SELECT
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domainWithoutWWW(URL) AS domain,
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domainWithoutWWW(REFERRER_URL) AS referrer,
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device_type,
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count() cnt
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FROM hits
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GROUP BY domain, referrer, device_type
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ORDER BY cnt DESC
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LIMIT 5 BY domain, device_type
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LIMIT 100
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```
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