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48 lines
3.1 KiB
Markdown
48 lines
3.1 KiB
Markdown
---
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slug: /en/sql-reference/statements/select/from
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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 ReplacingMergeTree, SummingMergeTree, AggregatingMergeTree, CollapsingMergeTree and VersionedCollapsingMergeTree tables.
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`SELECT` queries with `FINAL` are executed in parallel. The [max_final_threads](../../../operations/settings/settings.md#max-final-threads) setting limits the number of threads used.
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There are drawbacks to using `FINAL` (see below).
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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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`FINAL` requires additional compute and memory resources, as the processing that normally would occur at merge time must occur in memory at the time of the query. However, using FINAL is sometimes necessary in order to produce accurate results, and is less expensive than running `OPTIMIZE` to force a merge. It is also sometimes possible to use different queries that assume the background processes of the `MergeTree` engine haven’t happened yet and deal with it by applying aggregation (for example, to discard duplicates). If you need to use FINAL in your queries in order to get the required results, then it is okay to do so but be aware of the additional processing required.
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`FINAL` can be applied automatically using [FINAL](../../../operations/settings/settings.md#final) setting to all tables in a query using a session or a user profile.
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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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