that clearly identifies a data field of the existing table. You can refer to the primary key from other tables to access the data of this table. All data fields referring to this primary key will be identified as an external key.
，以明确标识现有表格的某个数据字段。然后可以从其他表格中引用该主关键字来读取该表格中的数据。引用主关键字的所有数据字段将被标识为外部关键字。