When any of the following are true, the Precondition/Filter expression does not qualify as a query filter:

  1. If it does not contain at least one alias that was extended to the database.
  2. If it contains any attributes of Boolean data type. Boolean data types are implemented inconsistently in commercial RDBMS, and cannot be included in query filters.
  3. If it has relational operators with Boolean operands.
  4. If it uses an operator not supported by databases (see the next topic)
  5. If it references more than one alias not extended to the database.