ClickHouse/docs/en/engines/table-engines/special/join.md

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---
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slug: /en/engines/table-engines/special/join
sidebar_position: 70
sidebar_label: Join
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---
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# Join Table Engine
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Optional prepared data structure for usage in [JOIN](../../../sql-reference/statements/select/join.md#select-join) operations.
:::note
This is not an article about the [JOIN clause](../../../sql-reference/statements/select/join.md#select-join) itself.
:::
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## Creating a Table {#creating-a-table}
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``` sql
CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1] [TTL expr1],
name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2] [TTL expr2],
) ENGINE = Join(join_strictness, join_type, k1[, k2, ...])
```
See the detailed description of the [CREATE TABLE](../../../sql-reference/statements/create/table.md#create-table-query) query.
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## Engine Parameters
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### join_strictness
`join_strictness` [JOIN strictness](../../../sql-reference/statements/select/join.md#select-join-types).
### join_type
`join_type` [JOIN type](../../../sql-reference/statements/select/join.md#select-join-types).
### Key columns
`k1[, k2, ...]` Key columns from the `USING` clause that the `JOIN` operation is made with.
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Enter `join_strictness` and `join_type` parameters without quotes, for example, `Join(ANY, LEFT, col1)`. They must match the `JOIN` operation that the table will be used for. If the parameters do not match, ClickHouse does not throw an exception and may return incorrect data.
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## Specifics and Recommendations {#specifics-and-recommendations}
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### Data Storage {#data-storage}
`Join` table data is always located in the RAM. When inserting rows into a table, ClickHouse writes data blocks to the directory on the disk so that they can be restored when the server restarts.
If the server restarts incorrectly, the data block on the disk might get lost or damaged. In this case, you may need to manually delete the file with damaged data.
### Selecting and Inserting Data {#selecting-and-inserting-data}
You can use `INSERT` queries to add data to the `Join`-engine tables. If the table was created with the `ANY` strictness, data for duplicate keys are ignored. With the `ALL` strictness, all rows are added.
Main use-cases for `Join`-engine tables are following:
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- Place the table to the right side in a `JOIN` clause.
- Call the [joinGet](../../../sql-reference/functions/other-functions.md#joinget) function, which lets you extract data from the table the same way as from a dictionary.
### Deleting Data {#deleting-data}
`ALTER DELETE` queries for `Join`-engine tables are implemented as [mutations](../../../sql-reference/statements/alter/index.md#mutations). `DELETE` mutation reads filtered data and overwrites data of memory and disk.
### Limitations and Settings {#join-limitations-and-settings}
When creating a table, the following settings are applied:
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#### join_use_nulls
[join_use_nulls](../../../operations/settings/settings.md#join_use_nulls)
#### max_rows_in_join
[max_rows_in_join](../../../operations/settings/query-complexity.md#settings-max_rows_in_join)
#### max_bytes_in_join
[max_bytes_in_join](../../../operations/settings/query-complexity.md#settings-max_bytes_in_join)
#### join_overflow_mode
[join_overflow_mode](../../../operations/settings/query-complexity.md#settings-join_overflow_mode)
#### join_any_take_last_row
[join_any_take_last_row](../../../operations/settings/settings.md#settings-join_any_take_last_row)
#### join_use_nulls
[persistent](../../../operations/settings/settings.md#persistent)
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The `Join`-engine tables cant be used in `GLOBAL JOIN` operations.
The `Join`-engine allows to specify [join_use_nulls](../../../operations/settings/settings.md#join_use_nulls) setting in the `CREATE TABLE` statement. [SELECT](../../../sql-reference/statements/select/index.md) query should have the same `join_use_nulls` value.
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## Usage Examples {#example}
Creating the left-side table:
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``` sql
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CREATE TABLE id_val(`id` UInt32, `val` UInt32) ENGINE = TinyLog;
```
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``` sql
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INSERT INTO id_val VALUES (1,11)(2,12)(3,13);
```
CLICKHOUSE-2720: progress on website (#865) * update presentations * CLICKHOUSE-2936: redirect from clickhouse.yandex.ru and clickhouse.yandex.com * update submodule * lost files * CLICKHOUSE-2981: prefer sphinx docs over original reference * CLICKHOUSE-2981: docs styles more similar to main website + add flags to switch language links * update presentations * Less confusing directory structure (docs -> doc/reference/) * Minify sphinx docs too * Website release script: fail fast + pass docker hash on deploy * Do not underline links in docs * shorter * cleanup docker images * tune nginx config * CLICKHOUSE-3043: get rid of habrastorage links * Lost translation * CLICKHOUSE-2936: temporary client-side redirect * behaves weird in test * put redirect back * CLICKHOUSE-3047: copy docs txts to public too * move to proper file * remove old pages to avoid confusion * Remove reference redirect warning for now * Refresh README.md * Yellow buttons in docs * Use svg flags instead of unicode ones in docs * fix test website instance * Put flags to separate files * wrong flag * Copy Yandex.Metrica introduction from main page to docs * Yet another home page structure change, couple new blocks (CLICKHOUSE-3045) * Update Contacts section * CLICKHOUSE-2849: more detailed legal information * CLICKHOUSE-2978 preparation - split by files * More changes in Contacts block * Tune texts on index page * update presentations * One more benchmark * Add usage sections to index page, adapted from slides * Get the roadmap started, based on slides from last ClickHouse Meetup * CLICKHOUSE-2977: some rendering tuning * Get rid of excessive section in the end of getting started * Make headers linkable * CLICKHOUSE-2981: links to editing reference - https://github.com/yandex/ClickHouse/issues/849 * CLICKHOUSE-2981: fix mobile styles in docs * Ban crawling of duplicating docs * Open some external links in new tab * Ban old docs too * Lots of trivial fixes in english docs * Lots of trivial fixes in russian docs * Remove getting started copies in markdown * Add Yandex.Webmaster * Fix some sphinx warnings * More warnings fixed in english docs * More sphinx warnings fixed * Add code-block:: text * More code-block:: text * These headers look not that well * Better switch between documentation languages * merge use_case.rst into ya_metrika_task.rst * Edit the agg_functions.rst texts * Add lost empty lines
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Creating the right-side `Join` table:
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``` sql
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CREATE TABLE id_val_join(`id` UInt32, `val` UInt8) ENGINE = Join(ANY, LEFT, id);
```
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``` sql
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INSERT INTO id_val_join VALUES (1,21)(1,22)(3,23);
```
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Joining the tables:
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``` sql
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SELECT * FROM id_val ANY LEFT JOIN id_val_join USING (id);
```
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``` text
┌─id─┬─val─┬─id_val_join.val─┐
│ 1 │ 11 │ 21 │
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│ 2 │ 12 │ 0 │
│ 3 │ 13 │ 23 │
└────┴─────┴─────────────────┘
```
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As an alternative, you can retrieve data from the `Join` table, specifying the join key value:
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``` sql
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SELECT joinGet('id_val_join', 'val', toUInt32(1));
```
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``` text
┌─joinGet('id_val_join', 'val', toUInt32(1))─┐
│ 21 │
└────────────────────────────────────────────┘
```
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Deleting a row from the `Join` table:
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```sql
ALTER TABLE id_val_join DELETE WHERE id = 3;
```
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```text
┌─id─┬─val─┐
│ 1 │ 21 │
└────┴─────┘
```