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100 lines
3.5 KiB
Markdown
100 lines
3.5 KiB
Markdown
# odbc {#table_functions-odbc}
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Returns table that is connected via [ODBC](https://en.wikipedia.org/wiki/Open_Database_Connectivity).
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```sql
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odbc(connection_settings, external_database, external_table)
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```
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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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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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The fields with the `NULL` values from the external table are converted into the default values for the base data type. For example, if a remote MySQL table field has the `INT NULL` type it is converted to 0 (the default value for ClickHouse `Int32` data type).
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## Usage example
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**Getting 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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Retrieving data from the MySQL table in ClickHouse:
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```sql
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SELECT * FROM odbc('DSN=mysqlconn', 'test', 'test')
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```
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```text
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┌─int_id─┬─int_nullable─┬─float─┬─float_nullable─┐
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│ 1 │ 0 │ 2 │ 0 │
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└────────┴──────────────┴───────┴────────────────┘
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```
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## See Also
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- [ODBC external dictionaries](../../query_language/dicts/external_dicts_dict_sources.md#dicts-external_dicts_dict_sources-odbc)
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- [ODBC table engine](../../operations/table_engines/odbc.md).
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[Original article](https://clickhouse.tech/docs/en/query_language/table_functions/jdbc/) <!--hide-->
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