ER-Model identifying relationships 與 non-identifying relationships的差別

https://www.geeksforgeeks.org/difference-between-identifying-and-non-identifying-relationships/

https://matthung0807.blogspot.com/2018/03/er-model-identifying-relationships-non.html

結論：簡單說：

實的關聯 (identifying relationships)：一個是母表 vs 子表，外部關聯鍵 foreign key的值，是不能為NULL，一定要有值！

虛的關聯 (non-identifying relationships)：foreign key 可以為 NULL