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    ###### tags: `Data Base` # 第九章: ER/EER Relational Mapping ## ER 轉 Relational Mapping Algorithm [Toc] ### Step 1: Mapping of Regular Entity Types - 把 strong entity type E 變成 relation table R ,並包含所有 E 的 attributes - 選一個 key,若選到 composite attribute,則全部為 primary key ![](https://i.imgur.com/TmCEFYa.png) ### Step 2: Mappingg of Weak Entity Types - 把 weak entity type W 轉成 relation R 並且將 all attributes 包含於 R 中 - 取 owner 的 primary key 當作 FK 放入 R 中 - R 的 PK 是 FK + partial key ![](https://i.imgur.com/sawuMSz.png) ### Step 3: Mapping of Binary 1:1 Relation Types - 3 種模式: - 1. Foreign Key approach: - 選一個有關係的 entity ,並把他的 PK 當成 FK include 進另一個 entity。(include 別人)的 entity 最好是 total 關係。 - 2. Merged relation option: - 將兩個 entity 與之間的 relation 合併成一個表,適合於兩個 entity 都是 total - 3. Cross-reference or relationship relation option: - 建立新的表來代表 relation,使用兩方 entity 的 PK 來參考 - 第一種 ![](https://i.imgur.com/iyH2UtI.png) ### Step 4: Mapping of Binary 1:N Relationship Types - 把 1 的那方的 PK 當作 FK include 進另一個 entity 中 - Include attribute 到 N 的 entity ![](https://i.imgur.com/gChmdLs.png) ### Step 5: Mapping of Binary M:N Relationshop Types - 建立新的表代表 relationship ,包含兩方的 PK ![](https://i.imgur.com/ANDvKK4.png) ### Step 6: Mapping of Multivalued attributes - 建立新的表代表多值,並用 entity 的 PK 變成 FK 與多值組合成這個表的 PK ![](https://i.imgur.com/bXLmLFt.png) ### Step 7: Mapping of N-ary Rekationship Types - 當 N > 2,建立新的表來表示 relation ,並將多方 entity PK 包含進這個表 ![](https://i.imgur.com/IMIpDUE.png) ### Step 8: Mapping Specialization or Generalization - 進行 specailization 與 generalized,有四個 options: - 8A(specialize): 每個 subclass/superclass 都建一個表,並將各個連接的 attribute 填入表中,另外,subclass 需 include superclass 的 PK 並成為 subclass 表的 PK ![](https://i.imgur.com/q7F8p8c.png) Example ![](https://i.imgur.com/M1yaeua.png) ![](https://i.imgur.com/pbDMM8X.png) - 8B(genralize): 每個 subclass 都建一個表,並將各個 attribute 填入表中,另外,subclass 們需要共有的 attributes 會填入 subclass 中,並找出一個共同的 PK ![](https://i.imgur.com/ZUz347G.png) Example ![](https://i.imgur.com/V9W5ivC.png) ![](https://i.imgur.com/MsxKWrC.png) - 8C(for disjoint specialize): 只建立一個 superclass 的表,並將各個 subclass 的 attribute 納入 superclass 的表中,另外,加上一個 type 的 attribute 來表示是哪個 subclass ![](https://i.imgur.com/WLmBsNK.png) Example ![](https://i.imgur.com/bto26pF.png) ![](https://i.imgur.com/s840sTz.png) - 8D(for overlapping specialize): 只建立一個 superclass 的表,並加入 type 的 boolean 值,若為該 subclass ,則布林值為 true ,並在旁邊加上各個 subclass 的 attribute ![](https://i.imgur.com/27348wP.png) Example ![](https://i.imgur.com/SmqHTh5.png) ![](https://i.imgur.com/QC8ZZxf.png) - 多重繼承狀況: 使用 8D 的狀況處理 Example ![](https://i.imgur.com/kRD8q3X.png) ![](https://i.imgur.com/wZwA9Rb.png) ### Step 9: Mapping of Union Types - 建議 subclass 的表,並加入共同的 attributes - 定義一個新的 key attribute: 替代鍵 Example: Registered_vehicle ![](https://i.imgur.com/MNFswUU.png)

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