1. A computer-implemented method comprising: providing Structured Query Language (SQL) extensions for a relational model of a database described by SQL for use in coding database queries to recognize features of an entity-relationship model (ERM) for an application that stores data in the database, wherein the SQL extensions include typed data structures for defining entities of the application, associations defining relationships between the entities of the application, and for defining annotations for data models of the application, wherein the annotations enable defining additional metadata that extends data definitions of an existing data model of the application without requiring developing a new data model or redundant type structures for the application; in a database query engine configured to query the database: receiving a database query communicated to the database query engine, wherein the database query is coded in SQL with the SQL extensions; translating the database query in the database query engine to query the database; and receiving a query result from the database, wherein the existing data model of the application and the additional metadata defined by the extensions of SQL are processed together at runtime in the database query engine.