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    ###### tags: `Data Base` # Data Base 第三章 ## 資料模型: 包含 Data, Relation, constraint。 ## 三種不同資料模型: ### 概念資料模型 - 以幾何方式描述使用者認知資料 - 使用幾何圖形符號建立模型 ### 邏輯資料模型 - 使用 4 種資料庫類型建立資料模型 ### 實體資料模型 - 實體建構資料 ## 概念資料模型: ### 實體關聯模型: - 以幾何圖形建立資料庫結構圖 - 畫出來的圖稱之為[實體關聯圖] - Base on Entity and Relation ## 邏輯資料模型: #### 階層式資料庫模型: - 資料結構:階層式樹狀結構,使用父子關聯指標連接'紀錄集合' 父紀錄多個子節點,子記錄一個父節點 - 有兩種基本集合型態: - 紀錄型態:一組欄位組成,每筆資料稱為 '紀錄',實際資料式若干比紀錄構成 - 父子關聯型態:兩個'紀錄型態'間連接型態,為一對多 #### 網路式資料庫模型: - 資料結構:使用'連接'來連結'紀錄集合',父與 子節點,為多對多 - 有兩種基本集合型態: - 紀錄型態: - 連接型態:描述兩個紀錄型態的關係,屬於一對多 #### 關聯式資料庫模型: - 定義:一組關聯表的集合 資料結構: - 關聯表綱要:包含關聯表之表名,屬性,定義域 - 關聯表實例:儲存數據,每筆資料都是一個 tuple - 資料操作或運算:新增,刪除,更新,查詢(關聯式係數) - 完整性限制條件:維護資料完整性條件集,確保儲存資料 合理 - 術語: 維度:有幾個 attribute 基數: tuple 個數 主鍵(PK):一個或多個 attr 而成的單元,用於辨識關聯表 內唯一的紀錄 定義域:屬性的 Datatype - 關聯表綱要的表示語法 ``` SQL= relation_name (attribute1, attribute2, … , attributeN) ``` 屬性有加上實線底線的表示主鍵角色 屬性有加上虛線底線(不一定有)的表示外來鍵角色 - 如果在關聯表綱要裡,需要標示定義域,可以使用括號標示在屬性後 ``` SQL= Students (sid(int), name(char), address(var char), tel(char), birthday(date)) ``` - 關聯表實例: 若干個值組(Tuples)的集合,因為資料可能隨時更改,所以關聯表實例是指某一個時間點的值組集 - 空值(NULL Values): 1.未知,不適用,不知道,找不到的值 2.不是 0 ,不是空字串 3.沒意義,不能做真假比較 4.所有定義域包含空值 - 三種不同關聯表: - 基底關聯表: - 真正的儲存資料 - 真實關聯表 - 視界 - 無真正儲存資料 - 定義於其他基底關聯表的一種虛擬關聯表 - 快照 - 在某一時點的關聯表內容 - 有真正的儲存資料,屬於一種唯讀關聯表 - 完整限制條件: - 檢查輸入資料庫的資料,確保資料正確 - 避免關聯表之間資料不一致 - 防止不合法的資料存入 DB - 完整限制條件: - 鍵限制條件 - 鍵: - 單一或一組屬性的集合 - 超鍵: - 單一或一組屬性集合 - 超鍵滿足唯一性,即沒有2個超鍵值相同 - 透過超鍵的辨識,就可以在關聯表存取指定的值 - 候選鍵: - 每個關聯表至少有一個候選鍵 - 候選鍵是一個具有最少屬性的超鍵 - 候選鍵中沒有一個屬性可以刪除 - 單一屬性的超鍵一定是候選鍵 - 主鍵: - 從各候選鍵中選出其一,只有一個 - 辨識某比特定資料 - 主鍵不能維空值 - 盡可能選單一屬性為主鍵 - 替代鍵: - 不是主見的其他候選鍵稱為替代鍵 - 用於替代主鍵 - 外來鍵: - 單一屬性或一組屬性集合 - 參考其他關連表的主鍵 - 外來鍵與所參考主鍵定義域一樣,但屬性名可不同 - 外來鍵值可以為空值 - 同一個關聯表可以多個外來鍵 - 定義域限制條件 - 設定該屬性的 Datatype - 實體完整性 - 關聯表只能有一個主鍵 - 主鍵的任何部分都不可以是空值 - 主鍵值不能重複 - 使用主鍵值可以找到特定一筆資料 - 參考完整性 - 關聯表可以有多個外來鍵 - 外來鍵值可以是空值 - 外來鍵值若非空值,一定是來自所參考的關聯表的主鍵值 - 主鍵值的更新與刪除: - 限制性處理方式(Restrict):主鍵值有被外來鍵值參考時,拒絕更新或刪除操作 - 連鎖性處理方式(Cascade):主鍵值允許更新或刪除操作,並連鎖反應到參考的外來鍵值 - 空值化處理方式(Nullify):主鍵值允許更新或刪除操作,並將所有參考的外來鍵值都設為空值 - 關聯式資料庫綱要:一組關聯表綱要 + 各綱要附屬的完整性限制條件 #### 物件導向式資料庫模型 ## 實體資料模型: 針對特定 DB system 建立實際架構的模型 資料模型顯示資料如何實作和儲存在 DB

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