A conceptual schema is a design model used to plan out or visually represent the structure of information contained in a database or other computer system entity. It acts to delineate the specific entities in the system, along with their attributes, and the relationships between various entities. The purpose of a conceptual schema is to provide a higher-level order to a computing system. Computer systems in large scale organizations are so complex that they require this type of macro-management in order to keep them functioning properly and efficiently.
To visualize a conceptual schema, picture a series of squares. Each square represents an entity table in the structure of the system, such as Employees, Current Employees, Projects, Payscale, and so on. These entities are arranged from top to bottom and connected with lines depending on relationships and associations. For example, the Current Employees table will naturally be a related subset of the Employees table, so the Current Employees table will branch vertically from the Employees table. Conversely, the Projects and Payscales table will be related to the Current Employees table, as each current employee will both be receiving a paygrade and working on one or more projects As a result, these tables will branch horizontally from the Current Employees table.
In addition to elaborating upon basic relationships between entities, a conceptual schema can also describe parameters that must be satisfied in the system. These are organization specific rules implemented to enforce order in the system. For example, specifying that each Current Employee must also exist in the Employees table, or specifying that each Current Employees must have at least one but not more than three projects. By implementing these rules at a design level, the company ensures that they cannot be violated in the system.
This structure allows system designers to instantly have an understanding of the various parts and subsystems of a computer network. The end result of this is that changes to the system become easier to put into place, as it is apparent which related entities will be affected by modification to specific entities in the system. Continuing the previous example, from the dependent relationship between Employees and Current Employees, it becomes obvious that any changes to the Employees entity will naturally affect the Current Employees entity as well.