Stream-Oriented Query Languages and Operators
Continuous query languages
Many research prototypes and commercial products have emerged in the new area of stream processing. All of these systems support a language for specifying queries. A fundamental difference between a stream query language and a conventional query language like SQL is that stream queries are not one-time computations, but rather, they continue to produce answers as new tuples arrive on one or more input streams. Thus, queries are registered with the system and answers continue to evolve over time. This new assumption is crucial to understanding some of the technical differences that arise in stream query languages.
Most stream query languages try to extend SQL in one way or another. The form of these extensions can be either a purely textual extension of SQL or GUI, through which users can construct dataflow diagrams that connect extended versions of relational operators. These days, many systems provide both.
The most fundamental addition...
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