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81 lines
1.7 KiB
Markdown
81 lines
1.7 KiB
Markdown
---
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slug: /zh/sql-reference/statements/select/with
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sidebar_label: WITH
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---
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# WITH子句 {#with-clause}
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本节提供对公共表表达式的支持 ([CTE](https://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL)），所以结果 `WITH` 子句可以在其余部分中使用 `SELECT` 查询。
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## 限制 {#limitations}
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1. 不支持递归查询。
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2. 当在section中使用子查询时，它的结果应该是只有一行的标量。
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3. Expression的结果在子查询中不可用。
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## 例 {#examples}
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**示例1:** 使用常量表达式作为 “variable”
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``` sql
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WITH '2019-08-01 15:23:00' as ts_upper_bound
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SELECT *
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FROM hits
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WHERE
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EventDate = toDate(ts_upper_bound) AND
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EventTime <= ts_upper_bound
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```
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**示例2:** 从SELECT子句列表中逐出sum(bytes)表达式结果
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``` sql
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WITH sum(bytes) as s
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SELECT
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formatReadableSize(s),
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table
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FROM system.parts
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GROUP BY table
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ORDER BY s
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```
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**例3:** 使用标量子查询的结果
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``` sql
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/* this example would return TOP 10 of most huge tables */
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WITH
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(
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SELECT sum(bytes)
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FROM system.parts
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WHERE active
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) AS total_disk_usage
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SELECT
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(sum(bytes) / total_disk_usage) * 100 AS table_disk_usage,
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table
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FROM system.parts
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GROUP BY table
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ORDER BY table_disk_usage DESC
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LIMIT 10
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```
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**例4:** 在子查询中重用表达式
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作为子查询中表达式使用的当前限制的解决方法，您可以复制它。
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``` sql
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WITH ['hello'] AS hello
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SELECT
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hello,
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*
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FROM
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(
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WITH ['hello'] AS hello
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SELECT hello
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)
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```
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``` text
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┌─hello─────┬─hello─────┐
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│ ['hello'] │ ['hello'] │
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└───────────┴───────────┘
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```
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