September 2013
Intermediate to advanced
264 pages
5h
Chinese
Content preview from Bad Data 技術手冊
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儲存圖形
|
171
我們不需要實體擁有不同的型態來表示頂點與邊。我們只需要在頂點上包含一個
type
的性質來表示該頂點代表什麼，舉例來說，可能是資產或是服務。我們能夠在邊上使用
標籤，來讓每個邊代表一個分配。我們能夠用邊上的性質來表示分配的百分比以及有效
的時間，也可以表示其他會有需要的性質。
現在我們只剩下頂點可以被分配給另一個頂點，而從在 ER 圖中的模型簡化的綱要則變
為如圖 13-3 所示。
Vertex
id:
name:
type:
Edge
label: allocated_to
id:
percentage:
eective_date:
圖 13-3 從 ER 圖中的模型簡化
儲存圖形
當我們決定要使用圖形資料結構來為我們的領域建立模型時，我們必須決定要如何維持
資料。我們能夠繼續使用一個關聯式資料庫來表示圖形，也許是將分配的清單儲存成邊
的連接清單。然而，SQL 是一個集合導向的語言，用來表達圖形概念並不是它的強項。
不論綱要為何，使用 SQL 以及關聯式資料庫的技術來解決圖形問題，容易導致以關聯
式資料庫為後端的物件導向系統中，常見的阻抗不匹配（impedance mismatch）問題。
阻抗不匹配源自於電機領域，它是一個用來搭配來源與負載阻抗的程序，主要是用來將
功率最大化以及將在傳輸線中的訊號反射最小化。而物件—關聯的阻抗不匹配則是一個
譬喻，套用在當關聯式資料庫綱要（來源），跟如 Java 或是 Ruby（負載）等物件導向
程式語言中的目標物件模型不一致的狀況時。這時都會需要一個在兩者（訊號反射∕功 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.