Reset the date portion to the first of the month while preserving the time

Question

Is there any way to reset the date portion to the first of the month while preserving the time? For example:

2018-01-02 23:00:00 -> 2018-01-01 23:00:00
2018-04-04 10:00:00 -> 2018-04-01 10:00:00

Answer 1

with x as (
  select to_date( '2018-01-02 23:00:00', 'yyyy-mm-dd hh24:mi:ss') as d from dual
  union all
  select to_date(  '2018-04-04 10:00:00', 'yyyy-mm-dd hh24:mi:ss') from dual
)


SELECT d, d - trunc( d ) + trunc( d, 'MM' )
FROM x;

D                   D-TRUNC(D)+TRUNC(D,
------------------- -------------------
2018-01-02 23:00:00 2018-01-01 23:00:00
2018-04-04 10:00:00 2018-04-01 10:00:00