Unlike SQL queries executed in a traditional RDBMS, which return a result and exit, SQL queries executed in a RDSMS do not exit, generating results continuously as new data become available.
The most common RDSMS SQL query is performed with the declarative SELECT statement.
A continuous SQL SELECT operates on data across one or more data streams, with optional keywords and clauses that include FROM with an optional JOIN subclause to specify the rules for joining multiple data streams, the WHERE clause and comparison predicate to restrict the records returned by the query, GROUP BY to project streams with common values into a smaller set, HAVING to filter records resulting from a GROUP BY, and ORDER BY to sort the results.
RDSMS SQL queries also operate on data streams over time or row-based windows.
The following example shows a second continuous SQL query using the WINDOW clause with a one-second duration.