A diagram is (i) a set of symbols selected in an alphabet of elementary shapes, such as rectangles, circles, and complex shapes, built by composing elementary shapes, and (ii) a set of connections linking symbols, such as straight lines, curved lines, directed lines, etc. Diagrams are used to visualize in all aspects of data base design and usage a wide set of concepts, artifacts, schemas, values, such as conceptual schemas, logical schemas, UML diagrams, database instances, queries, results of queries.
The visual representation made possible by a diagram expresses a functional relationship between concepts represented and symbols/connections representing them. e.g., in the diagrammatic representation of Entity Relationship schemas, a rectangle is associated to an entity, a diamond is associated to a relationship.
When drawing a diagram, human beings and display devices adopt suitable drawing conventions, aesthetic...