alter table 表 add constraint fk_name foreign key(约束列) references 主表(约束列) on update cascade on delete no action 在创建表的时候就需要设置级联删除和级联更新（修改）。
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In this example, we will use the name C0989623 as the name of the foreign key (Figure 6).
在本例中，我们将使用名称C0989623作为外键的名称(图6)。
The name attribute defines the foreign key on the table to which the Customer class is mapped, and the referenced column defines the primary key to which the table CustomerOrder is mapped.
名称属性在customer类映射到的表上定义外键，参考列定义Table CustomerOrder映射到的主键。
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