Relational query languages have enabled the programmer to express queries using a logical model of data without any knowledge of the underlying physical structures. To help applications realize the benefits of such declarative querying of data fully, there has been much work along the following three dimensions:
- Application programming interfaces (for example, ODBC, JDBC) have been developed to enable applications connect to and access data in a relational database system. However, when connecting using these interfaces, the application programmer must still handle two different programming models. Language integrated query (LINQ) is an elegant example of integration where query expressions are introduced as a first-class citizen in the programming languages to avoid the above problem, and a mapping tool (LINQ to SQL) translates language-integrated queries into SQL for the database backend. More recently, databases have been exposing a REST API for the ease of mobile and web applications.
- Modern database systems provide extensibility so that applications programmers are not limited to using the built-in types and functions in SQL. All major database systems support user-defined functions that may be used in selection, aggregation, or table expressions in a query. These user-defined functions (potentially with parameters) are written in native SQL or programming languages for which the database server provides runtime support. Such extensibility mechanisms have been used by database systems to add support for data types such as geospatial.
- The SQL standard has added new operators and constructs to make declarative querying in relational languages more convenient or expressive, for example, recursion, window functions, grouping sets, within group.
Despite the advances that have already taken place along these three dimensions, there continues to be proposals from time to time to further enrich functionality of relational databases to support important classes of applications.
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