Joe Celko's Trees and Hierarchies in SQL for Smarties

CHAPTER 1 Graphs, Trees, and Hierarchies

LET’S START WITH a little mathematical background. Graph theory is a branch of mathematics that deals with abstract structures, known as graphs. These are not the presentation charts that you get out of a spreadsheet package.

Very loosely speaking, a graph is a diagram of “dots” (called nodes or vertices) and “lines” (edges) that model some kind of “flow” or relationship. The edges can be undirected or directed. Graphs are very general models. In circuit diagrams the edges are the wires and the nodes are the components. On a road map the nodes are the towns and the edges are the roads. Flowcharts, organizational charts, and a hundred other common abstract models you see every day are all shown as graphs. ...