A Semantics for a Query Language over Sensors, Streams and Relations

We introduce a query language over sensors, streams and relations and formally describe its semantics. Although the language was specifically designed for sensor network querying, where data is pulled into streams, the semantics contributed in the paper also encompasses the case in which data is pushed onto streams or else lies stored in classical relations. The approach taken is that continuous queries over streams are an extension of classical queries over stored extents. Apart from the fact that query evaluation over streams is reactive, or periodic, the main difference is the conception of windows as an additional collection type with the consequent use of type converter operations to and from streams and windows (which, as bounded collections of tuples, can be operated on in a relational-algebraic setting). The language and the semantics we provide for it advance on previous work in being more comprehensive with respect to the collection types allowed and in being more flexible as to the number and content of the windows contributing to the result at each evaluation event of a continuous query. The formalization advances on previous work in clarifying the implementation onus.