A phrase that defines specific conditions that individual records
    must meet before Oracle Rdb includes them in a record stream. A
    record stream is a temporary group of related records that
    satisfy the conditions you specify in the record selection
    expression. A record stream can consist of:

    o  All the records of one or more relations

    o  A subset of the records in one or more relations

    The following sections describe how to use the record selection
    expression to form record streams.