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* Update of english version of descriprion of the table function `file`. * New syntax for ReplacingMergeTree. Some improvements in text. * Significantly change article about SummingMergeTree. Article is restructured, text is changed in many places of the document. New syntax for table creation is described. * Descriptions of AggregateFunction and AggregatingMergeTree are updated. Russian version. * New syntax for new syntax of CREATE TABLE * Added english docs on Aggregating, Replacing and SummingMergeTree. * CollapsingMergeTree docs. English version. * 1. Update of CollapsingMergeTree. 2. Minor changes in markup * Update aggregatefunction.md * Update aggregatefunction.md * Update aggregatefunction.md * Update aggregatingmergetree.md * GraphiteMergeTree docs update. New syntax for creation of Replicated* tables. Minor changes in *MergeTree tables creation syntax. * Markup fix * Markup and language fixes * Clarification in the CollapsingMergeTree article * Clarifications for MySQL table engine and toStartOfMonth function
35 lines
1.7 KiB
Markdown
35 lines
1.7 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 on_duplicate_clause` expression to the `INSERT` query.
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Example: `INSERT INTO t (c1,c2) VALUES ('a', 2) ON DUPLICATE KEY UPDATE c2 = c2 + 1`, where `on_duplicate_clause` is `UPDATE c2 = c2 + 1`. See MySQL documentation to find which `on_duplicate_clause` you can use with `ON DUPLICATE KEY` clause.
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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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[Original article](https://clickhouse.yandex/docs/en/operations/table_engines/mysql/) <!--hide-->
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