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33 lines
2.4 KiB
Markdown
33 lines
2.4 KiB
Markdown
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# MySQL
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Движок MySQL позволяет выполнять `SELECT` запросы над данными, хранящимися на удалённом MySQL сервере.
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Формат вызова:
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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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**Параметры вызова**
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- `host:port` — Адрес сервера MySQL.
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- `database` — Имя базы данных на сервере MySQL.
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- `table` — Имя таблицы.
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- `user` — Пользователь MySQL.
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- `password` — Пароль пользователя.
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- `replace_query` — Флаг, устанавливающий замену запроса `INSERT INTO` на `REPLACE INTO`. Если `replace_query=1`, то запрос заменяется.
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- `on_duplicate_clause` — Добавляет выражение `ON DUPLICATE KEY 'on_duplicate_clause'` в запрос `INSERT`.
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Например, `INSERT INTO t (c1,c2) VALUES ('a', 2) ON DUPLICATE KEY UPDATE c2 = c2 + 1`, где `on_duplicate_clause` — `UPDATE c2 = c2 + 1`. Какие выражения `on_duplicate_clause` вы можете использовать с `ON DUPLICATE KEY`, смотрите в документации MySQL.
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Чтобы указать `on_duplicate_clause` необходимо передать `0` в параметр `replace_query`. Если одновременно передать `replace_query = 1` и `on_duplicate_clause`, то ClickHouse сгенерирует исключение.
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На данный момент простые условия `WHERE`, такие как `=, !=, >, >=, <, <=` будут выполняться на стороне сервера MySQL.
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Остальные условия и ограничение выборки `LIMIT` будут выполнены в ClickHouse только после выполнения запроса к MySQL.
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Движок `MySQL` не поддерживает тип данных [Nullable](../../data_types/nullable.md), поэтому при чтении данных из таблиц MySQL `NULL` преобразуются в значения по умолчанию для заданного типа столбца, обычно это 0 или пустая строка.
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[Оригинальная статья](https://clickhouse.yandex/docs/ru/operations/table_engines/mysql/) <!--hide-->
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