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71 lines
2.3 KiB
Markdown
71 lines
2.3 KiB
Markdown
---
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toc_title: LIMIT BY
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---
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# LIMIT BY子句 {#limit-by-clause}
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与查询 `LIMIT n BY expressions` 子句选择第一个 `n` 每个不同值的行 `expressions`. `LIMIT BY` 可以包含任意数量的 [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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在进行查询处理时，ClickHouse选择按排序键排序的数据。排序键设置显式地使用一个[ORDER BY](order-by.md#select-order-by)条款或隐式属性表的引擎(行顺序只是保证在使用[ORDER BY](order-by.md#select-order-by),否则不会命令行块由于多线程)。然后ClickHouse应用`LIMIT n BY 表达式`，并为每个不同的`表达式`组合返回前n行。如果指定了`OFFSET`，那么对于每个属于不同`表达式`组合的数据块，ClickHouse将跳过`offset_value`从块开始的行数，并最终返回最多`n`行的结果。如果`offset_value`大于数据块中的行数，则ClickHouse从数据块中返回零行。
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!!! note "注"
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`LIMIT BY` 是不相关的 [LIMIT](../../../sql-reference/statements/select/limit.md). 它们都可以在同一个查询中使用。
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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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该 `SELECT * FROM limit_by ORDER BY id, val LIMIT 2 OFFSET 1 BY id` 查询返回相同的结果。
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以下查询返回每个引用的前5个引用 `domain, device_type` 最多可与100行配对 (`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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