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28 lines
1.4 KiB
Markdown
28 lines
1.4 KiB
Markdown
# MySQL
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MySQL 引擎可以对存储在远程 MySQL 服务器上的数据执行 `SELECT` 查询。
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调用格式：
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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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**调用参数**
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- `host:port` — MySQL 服务器地址。
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- `database` — 数据库的名称。
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- `table` — 表名称。
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- `user` — 数据库用户。
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- `password` — 用户密码。
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- `replace_query` — 将 `INSERT INTO` 查询是否替换为 `REPLACE INTO` 的标志。如果 `replace_query=1`，则替换查询
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- `'on_duplicate_clause'` — 将 `ON DUPLICATE KEY UPDATE 'on_duplicate_clause'` 表达式添加到 `INSERT` 查询语句中。例如：`impression = VALUES(impression) + impression`。如果需要指定 `'on_duplicate_clause'`，则需要设置 `replace_query=0`。如果同时设置 `replace_query = 1` 和 `'on_duplicate_clause'`，则会抛出异常。
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此时，简单的 `WHERE` 子句（例如 ` =, !=, >, >=, <, <=`）是在 MySQL 服务器上执行。
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其余条件以及 `LIMIT` 采样约束语句仅在对MySQL的查询完成后才在ClickHouse中执行。
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`MySQL` 引擎不支持 [Nullable](../../data_types/nullable.md) 数据类型，因此，当从MySQL表中读取数据时，`NULL` 将转换为指定列类型的默认值（通常为0或空字符串）。
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[Original article](https://clickhouse.yandex/docs/zh/operations/table_engines/mysql/) <!--hide-->
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