Confluent KSQL although excellent in exposing streaming kafka topics through a SQL-like interface, it is not fully ANSI SQL compliant. It has some differences because it is geared at processing streaming data.
You can refer to the full list of available functions here:
Note newer versions like 5.0 or 5.1 support more functions.
Arcadia will continue to integrate and expose the new functionality as Confluent KSQL matures.