registerFunction( "substring", new SQLFunctionTemplate( StandardBasicTypes.STRING, "substring(?1, ?2, ?3)" ) );

|--SUBSTRING--(--source_string--FROM--start_position--+-------------+--)--|
                                                      '-FOR--length-'      

SELECT SUBSTRING('ABCDEFG' FROM 3 FOR 2) FROM mytable;

|--SUBSTR--(--source_string--,--start_position--+-----------+--)--|
                                                '-,--length-'      

SELECT SUBSTR('ABCDEFG', 0, 1)   FROM mytable;