A computer software language capable of expressing registered queries that operate on one more or more data streams continuously. The language of the present invention is based on a publish/subscribe model in that queries subscribe to data streams and publish to data streams. Also, the language of the present invention can express queries that operate directly on data streams. Since queries expressed in the language of the present invention may be executed continuously and directly on data streams, the language includes a clause for specifying time-based and/or row-based windows for the input data stream. Operations are then performed on the data within such windows. In one embodiment, the language is also SQL-like and includes a clause for defining named windows (which can be used in any number of queries); a clause for detecting a pattern, and correlated database subqueries for correlating data stream data with database tables.