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80 lines
1.7 KiB
Markdown
80 lines
1.7 KiB
Markdown
---
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toc_title: WITH
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---
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# WITH Clause {#with-clause}
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This section provides support for Common Table Expressions ([CTE](https://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL)), so the results of `WITH` clause can be used in the rest of `SELECT` query.
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## Limitations
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1. Recursive queries are not supported.
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2. When subquery is used inside WITH section, it’s result should be scalar with exactly one row.
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3. Expression’s results are not available in subqueries.
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## Examples
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**Example 1:** Using constant expression as “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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**Example 2:** Evicting sum(bytes) expression result from SELECT clause column list
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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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**Example 3:** Using results of scalar subquery
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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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**Example 4:** Re-using expression in subquery
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As a workaround for current limitation for expression usage in subqueries, you may duplicate it.
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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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