A database schema diagram is a diagram that contains the entities and attributes that define the schema. A database schema diagram merely depicts the database design. It does not display the database's actual data. A schema can be a single table or a collection of related tables. A database schema is the logical configuration of an entire or portion of a relational database. It can be represented visually, and a set of formulas known as integrity constraints govern a database. These formulas are written in a data definition language like SQL.