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29 lines
1.5 KiB
Markdown
29 lines
1.5 KiB
Markdown
<a name="table_engines-mysql"></a>
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# MySQL
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The MySQL engine allows you to perform `SELECT` queries on data that is stored on a remote MySQL server.
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Call format:
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```
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MySQL('host:port', 'database', 'table', 'user', 'password'[, replace_query, 'on_duplicate_clause']);
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```
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**Call parameters**
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- `host:port` — Address of the MySQL server.
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- `database` — Database name on the MySQL server.
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- `table` — Name of the table.
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- `user` — The MySQL User.
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- `password` — User password.
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- `replace_query` — Flag that sets query substitution `INSERT INTO` to `REPLACE INTO`. If `replace_query=1`, the query is replaced.
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- `'on_duplicate_clause'` — Adds the `ON DUPLICATE KEY UPDATE 'on_duplicate_clause'` expression to the `INSERT` query. For example: `impression = VALUES(impression) + impression`. To specify `'on_duplicate_clause'` you need to pass `0` to the `replace_query` parameter. If you simultaneously pass `replace_query = 1` and `'on_duplicate_clause'`, ClickHouse generates an exception.
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At this time, simple `WHERE` clauses such as ` =, !=, >, >=, <, <=` are executed on the MySQL server.
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The rest of the conditions and the `LIMIT` sampling constraint are executed in ClickHouse only after the query to MySQL finishes.
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The `MySQL` engine does not support the [Nullable](../../data_types/nullable.md#data_type-nullable) data type, so when reading data from MySQL tables, `NULL` is converted to default values for the specified column type (usually 0 or an empty string).
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