tailieunhanh - PATTERNS OF DATA MODELING- P19

PATTERNS OF DATA MODELING- P19: Models provide the means for building quality software in a predictable manner. Models let developers think deeply about software and cope with large size and complexity. Developers can think abstractly before becoming enmeshed in the details of writing code. Although models are beneficial, they can be difficult to construct. That is where patterns come in. Patterns provide building blocks that help developers construct models faster and better. | Undirected Graph Changing over Time Template 71 SELECT FROM Node as N INNER JOIN Node_Edge AS NE ON INNER JOIN Edge AS E ON WHERE aNodeID AND IS NULL OR aDate AND IS NULL OR aDate AND IS NULL OR aDate AND IS NULL OR aDate ORDER BY Figure Undirected graph changing over time SQL query. Find the edges that connect to a node. SELECT FROM Edge AS E INNER JOIN Node_Edge AS NE ON INNER JOIN Node AS N ON WHERE anEdgeID AND IS NULL OR aDate AND IS NULL OR aDate AND IS NULL OR aDate AND IS NULL OR aDate ORDER BY Figure Undirected graph changing over time SQL query. Find the nodes for an edge. Date means that a Node applies from the past. A null expirationDate means that a Node applies into the future. Example An extended model Figure could add the aspect of time to the LinkedIn example. Such a model could track the history of members and connections. From firsthand experience with LinkedIn I know this is a realistic example. Member data not shown in the model changes over time. Also links can come and go as users join and leave the Web site. 72 Chapter 4 Undirected Graph Template Undirected graph 1 July 2000 0100 Node table Undirected graph 1 July 2000 0300 Edge table node ID node Name effective Date expiration Date udg ID 1 A 1 2 B 1 July 2000 0100 1 3 C 1 4 D 1 5 E 1 6 F 1 7 G 1 July 2000 0300 1 edge ID edge Name effective Date expiration Date udg ID 51 c 1 52 d 1 53 e 1 54 f 1 July 2000 0100 1 55 g 1 56 h 1 57 i 1 58 j 1 July 2000 0300 1 Node_Edge table nodeID edgeID 1 51 3 51 1 52 4 52 1 53 5 53 2 54 5 54 Node_Edge table cont nodeID .