tailieunhanh - PATTERNS OF DATA MODELING- P13

PATTERNS OF DATA MODELING- P13: 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. | Structured Directed Graph Template 41 in Figure a can have child nodes each of which in turn can be a leaf node or a further branch node. The distinction between parent and child causes the sense of direction that effects directed edges. Note that with this template there can be at most one coupling between a pair of nodes. Figure adds the constraint that each node has a parent except for root nodes. The template itself is more permissive and lacks this constraint. In general a directed graph may have cycles but this template disallows them. A cycle starts at a node and after traversing a series of edges reaches the starting node. Since this template is based on a tree template it doesn t make sense to permit cycles. Each node may have a name. As Figure shows node names can be globally unique left template or unique within a context right template . a Globally unique node name I b Unique node name within a context Figure Structured directed graph UML template with node names. There are two variations of the template globally unique names and unique names within a context. IDEF1X Template Figure restates Figure with the IDEF1X notation. The following are foreign keys dgID references DG rootID references Node parentID references Branch childID references Node leafID references Node and branchID references Node. The generalization is ex-haustive every Node record must have a corresponding Leaf record or Branch record. The nodeDiscrim field is an enumeration with values Leaf and Branch indicating the subtype record. Figure defines parentID nodeName as the Node_Branch primary key but par-entID childID could be the primary key instead. Node_Branch defines node names for all nodes except for the roots. The root nodes obtain their name from the combination of dgID nodeName. Once again the choice of primary key is arbitrary for DG_Node and could have been the alternate key instead. SQL Queries Figure and Figure .