# 外连接

通过JOIN/ON关键字实现的连接统一使用`@Join`进行映射。

这类连接在 `ON` 关键字后添加动态查询条件。为此，引入新的查询对象映射 `ON` 子句的条件，并定义为主查询对象的字段。

为此，设计了新注解 `@Join`，用于映射连接表和指定连接类型，同时复用已有的 `@ForeignKey` 注解映射连接条件。

| LN | 对象代码                                                           | SQL 语句                         |
| -- | -------------------------------------------------------------- | ------------------------------ |
| 1  | public class CustomerOrdersQuery extends PageQuery {           |                                |
| 2  | @Join(from = @View(value = CustomerEntity.class, alias = "c"), | FROM customer c                |
| 3  | type = Join.JoinType.LEFT\_JOIN,                               | LEFT JOIN orders o             |
| 4  | join = @View(value = OrdersEntity.class, alias = "o"))         | ON o.o\_custkey = c.c\_custkey |
| 5  | private JoinOrders joinOrders;                                 | AND o.o\_comment NOT LIKE ?    |
| 6  | }                                                              |                                |


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