mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-12-14 02:12:21 +00:00
48 lines
2.7 KiB
Markdown
48 lines
2.7 KiB
Markdown
---
|
||
slug: /en/sql-reference/statements/select/from
|
||
sidebar_label: FROM
|
||
---
|
||
|
||
# FROM Clause
|
||
|
||
The `FROM` clause specifies the source to read data from:
|
||
|
||
- [Table](../../../engines/table-engines/index.md)
|
||
- [Subquery](../../../sql-reference/statements/select/index.md)
|
||
- [Table function](../../../sql-reference/table-functions/index.md#table-functions)
|
||
|
||
[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.
|
||
|
||
Subquery is another `SELECT` query that may be specified in parenthesis inside `FROM` clause.
|
||
|
||
`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.
|
||
|
||
## FINAL Modifier
|
||
|
||
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.
|
||
|
||
It is applicable when selecting data from ReplacingMergeTree, SummingMergeTree, AggregatingMergeTree, CollapsingMergeTree and VersionedCollapsingMergeTree tables.
|
||
|
||
`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.
|
||
|
||
There are drawbacks to using `FINAL` (see below).
|
||
|
||
### Drawbacks
|
||
|
||
Queries that use `FINAL` are executed slightly slower than similar queries that do not, because:
|
||
|
||
- Data is merged during query execution.
|
||
- Queries with `FINAL` read primary key columns in addition to the columns specified in the query.
|
||
|
||
**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).
|
||
|
||
`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.
|
||
|
||
## Implementation Details
|
||
|
||
If the `FROM` clause is omitted, data will be read from the `system.one` table.
|
||
The `system.one` table contains exactly one row (this table fulfills the same purpose as the DUAL table found in other DBMSs).
|
||
|
||
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.
|
||
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.
|