The term "database schema" can refer to a visual representation of a database, a set of rules that govern a database, or to the entire set of objects belonging to a particular user.
A database schema represents the logical configuration of all or part of a relational database. It can exist both as a visual representation and as a set of formulas known as integrity constraints that govern a database. These formulas are expressed in a data definition language, such as SQL. As part of a data dictionary, a database schema indicates how the entities that make up the database relate to one another, including tables, views, stored procedures, and more.
There are two main kinds of database schema:
- A logical database schema conveys the logical constraints that apply to the stored data. It may define integrity constraints, views, and tables.
- A physical database schema lays out how data is stored physically on a storage system in terms of files and indices.
What is sub schema?
A sub schema is a subset of the schema and inherits the same property that a schema has. The plan (or scheme) for a view is often called sub schema. Sub schema refers to an application programmer's (user's) view of the data item types and record types, which he or she uses.
