A schema is the structure of the database that defines the objects in the database (Fig. 1a). A database schema of a database system is its structure described in a formal language supported by the database management system (DBMS). The term "schema" refers to the organization of data as a blueprint of how the database is constructed (divided into database tables in the case of relational databases) (Fig. 1b). The formal definition of a database schema is a set of formulas (sentences) called integrity constraints imposed on a database. These integrity constraints ensure compatibility between parts of the schema.

A databases generally stores its schema in a data dictionary. Although a schema is defined in text database language, the term is often used to refer to a graphical depiction of the database structure.

Figure 1a. - select image to enlarge

A section of the SBL Schema

Figure 1b. - select image to enlarge

Here on the left, the SBL schema is defining the attributes "socialSecurityNumber" and "birthDate" of the SBL import file on the right. 

The purpose of an XML Schema is to define the legal building blocks of an XML document

1. the elements and attributes that can appear in a document
2. the number of (and order of) child elements
3. data types for elements and attributes
4. default and fixed values for elements and attributes