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73 lines
2.4 KiB
Markdown
73 lines
2.4 KiB
Markdown
---
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slug: /zh/sql-reference/statements/select/limit-by
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sidebar_label: LIMIT BY
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---
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# LIMIT BY子句 {#limit-by-clause}
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一个使用`LIMIT n BY expressions`从句的查询会以去重后的`expressions`结果分组，每一分组选择前`n`行。`LIMIT BY`指定的值可以是任意数量的[表达式](../../../sql-reference/syntax.md#syntax-expressions)。
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ClickHouse支持以下语法变体:
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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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处理查询时，ClickHouse首先选择经由排序键排序过后的数据。排序键可以显式地使用[ORDER BY](order-by.md#select-order-by)从句指定，或隐式地使用表引擎使用的排序键（数据的顺序仅在使用[ORDER BY](order-by.md#select-order-by)时才可以保证，否则由于多线程处理，数据顺序会随机化）。然后ClickHouse执行`LIMIT n BY expressions`从句，将每一行按 `expressions` 的值进行分组，并对每一分组返回前`n`行。如果指定了`OFFSET`，那么对于每一分组，ClickHouse会跳过前`offset_value`行，接着返回前`n`行。如果`offset_value`大于某一分组的行数，ClickHouse会从分组返回0行。
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:::note
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`LIMIT BY`与[LIMIT](../../../sql-reference/statements/select/limit.md)没有关系。它们可以在同一个查询中使用。
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:::
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## 例 {#examples}
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样例表:
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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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查询:
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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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与 `SELECT * FROM limit_by ORDER BY id, val LIMIT 2 OFFSET 1 BY id` 返回相同的结果。
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以下查询返回每个`domain,device_type`组合的前5个refferrer，总计返回至多100行(`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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