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131 lines
4.4 KiB
Markdown
131 lines
4.4 KiB
Markdown
---
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slug: /en/engines/table-engines/integrations/odbc
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sidebar_position: 2
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sidebar_label: ODBC
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---
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# ODBC
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Allows ClickHouse to connect to external databases via [ODBC](https://en.wikipedia.org/wiki/Open_Database_Connectivity).
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To safely implement ODBC connections, ClickHouse uses a separate program `clickhouse-odbc-bridge`. If the ODBC driver is loaded directly from `clickhouse-server`, driver problems can crash the ClickHouse server. ClickHouse automatically starts `clickhouse-odbc-bridge` when it is required. The ODBC bridge program is installed from the same package as the `clickhouse-server`.
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This engine supports the [Nullable](../../../sql-reference/data-types/nullable.md) data type.
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## Creating a Table {#creating-a-table}
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``` sql
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CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
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(
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name1 [type1],
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name2 [type2],
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...
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)
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ENGINE = ODBC(connection_settings, external_database, external_table)
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```
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See a detailed description of the [CREATE TABLE](../../../sql-reference/statements/create/table.md#create-table-query) query.
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The table structure can differ from the source table structure:
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- Column names should be the same as in the source table, but you can use just some of these columns and in any order.
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- Column types may differ from those in the source table. ClickHouse tries to [cast](../../../sql-reference/functions/type-conversion-functions.md#type_conversion_function-cast) values to the ClickHouse data types.
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- The [external_table_functions_use_nulls](../../../operations/settings/settings.md#external-table-functions-use-nulls) setting defines how to handle Nullable columns. Default value: 1. If 0, the table function does not make Nullable columns and inserts default values instead of nulls. This is also applicable for NULL values inside arrays.
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**Engine Parameters**
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- `connection_settings` — Name of the section with connection settings in the `odbc.ini` file.
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- `external_database` — Name of a database in an external DBMS.
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- `external_table` — Name of a table in the `external_database`.
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## Usage Example {#usage-example}
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**Retrieving data from the local MySQL installation via ODBC**
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This example is checked for Ubuntu Linux 18.04 and MySQL server 5.7.
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Ensure that unixODBC and MySQL Connector are installed.
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By default (if installed from packages), ClickHouse starts as user `clickhouse`. Thus, you need to create and configure this user in the MySQL server.
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``` bash
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$ sudo mysql
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```
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``` sql
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mysql> CREATE USER 'clickhouse'@'localhost' IDENTIFIED BY 'clickhouse';
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mysql> GRANT ALL PRIVILEGES ON *.* TO 'clickhouse'@'clickhouse' WITH GRANT OPTION;
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```
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Then configure the connection in `/etc/odbc.ini`.
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``` bash
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$ cat /etc/odbc.ini
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[mysqlconn]
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DRIVER = /usr/local/lib/libmyodbc5w.so
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SERVER = 127.0.0.1
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PORT = 3306
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DATABASE = test
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USERNAME = clickhouse
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PASSWORD = clickhouse
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```
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You can check the connection using the `isql` utility from the unixODBC installation.
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``` bash
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$ isql -v mysqlconn
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+-------------------------+
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| Connected! |
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...
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```
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Table in MySQL:
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``` text
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mysql> CREATE TABLE `test`.`test` (
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-> `int_id` INT NOT NULL AUTO_INCREMENT,
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-> `int_nullable` INT NULL DEFAULT NULL,
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-> `float` FLOAT NOT NULL,
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-> `float_nullable` FLOAT NULL DEFAULT NULL,
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-> PRIMARY KEY (`int_id`));
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Query OK, 0 rows affected (0,09 sec)
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mysql> insert into test (`int_id`, `float`) VALUES (1,2);
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Query OK, 1 row affected (0,00 sec)
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mysql> select * from test;
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+------+----------+-----+----------+
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| int_id | int_nullable | float | float_nullable |
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+------+----------+-----+----------+
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| 1 | NULL | 2 | NULL |
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+------+----------+-----+----------+
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1 row in set (0,00 sec)
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```
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Table in ClickHouse, retrieving data from the MySQL table:
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``` sql
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CREATE TABLE odbc_t
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(
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`int_id` Int32,
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`float_nullable` Nullable(Float32)
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)
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ENGINE = ODBC('DSN=mysqlconn', 'test', 'test')
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```
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``` sql
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SELECT * FROM odbc_t
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```
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``` text
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┌─int_id─┬─float_nullable─┐
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│ 1 │ ᴺᵁᴸᴸ │
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└────────┴────────────────┘
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```
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## See Also {#see-also}
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- [ODBC dictionaries](../../../sql-reference/dictionaries/external-dictionaries/external-dicts-dict-sources.md#dicts-external_dicts_dict_sources-odbc)
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- [ODBC table function](../../../sql-reference/table-functions/odbc.md)
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