If y is omitted, x is truncated to zero decimal places.
If y is negative, x is truncated to the left of the decimal point.
The following example displays truncating 5.75 to zero, 1, and - 1 decimal places:
SQL> SELECT TRUNC(5.75), TRUNC(5.75, 1), TRUNC(5.75, -1) FROM dual;
TRUNC(5.75) TRUNC(5.75,1) TRUNC(5.75,-1)
----------- ------------- --------------
5 5.7 0
SQL>