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46 lines
3.0 KiB
Markdown
46 lines
3.0 KiB
Markdown
---
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sidebar_label: FROM
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---
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# FROM Clause
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The `FROM` clause specifies the source to read data from:
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- [Table](../../../engines/table-engines/index.md)
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- [Subquery](../../../sql-reference/statements/select/index.md)
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- [Table function](../../../sql-reference/table-functions/index.md#table-functions)
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[JOIN](../../../sql-reference/statements/select/join.md) and [ARRAY JOIN](../../../sql-reference/statements/select/array-join.md) clauses may also be used to extend the functionality of the `FROM` clause.
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Subquery is another `SELECT` query that may be specified in parenthesis inside `FROM` clause.
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`FROM` clause can contain multiple data sources, separated by commas, which is equivalent of performing [CROSS JOIN](../../../sql-reference/statements/select/join.md) on them.
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## FINAL Modifier
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When `FINAL` is specified, ClickHouse fully merges the data before returning the result and thus performs all data transformations that happen during merges for the given table engine.
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It is applicable when selecting data from tables that use the [MergeTree](../../../engines/table-engines/mergetree-family/mergetree.md)-engine family. Also supported for:
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- [Replicated](../../../engines/table-engines/mergetree-family/replication.md) versions of `MergeTree` engines.
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- [View](../../../engines/table-engines/special/view.md), [Buffer](../../../engines/table-engines/special/buffer.md), [Distributed](../../../engines/table-engines/special/distributed.md), and [MaterializedView](../../../engines/table-engines/special/materializedview.md) engines that operate over other engines, provided they were created over `MergeTree`-engine tables.
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Now `SELECT` queries with `FINAL` are executed in parallel and slightly faster. But there are drawbacks (see below). The [max_final_threads](../../../operations/settings/settings.md#max-final-threads) setting limits the number of threads used.
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### Drawbacks
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Queries that use `FINAL` are executed slightly slower than similar queries that do not, because:
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- Data is merged during query execution.
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- Queries with `FINAL` read primary key columns in addition to the columns specified in the query.
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**In most cases, avoid using `FINAL`.** The common approach is to use different queries that assume the background processes of the `MergeTree` engine have’t happened yet and deal with it by applying aggregation (for example, to discard duplicates).
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## Implementation Details
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If the `FROM` clause is omitted, data will be read from the `system.one` table.
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The `system.one` table contains exactly one row (this table fulfills the same purpose as the DUAL table found in other DBMSs).
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To execute a query, all the columns listed in the query are extracted from the appropriate table. Any columns not needed for the external query are thrown out of the subqueries.
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If a query does not list any columns (for example, `SELECT count() FROM t`), some column is extracted from the table anyway (the smallest one is preferred), in order to calculate the number of rows.
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