How do I update just the day of a date value?

Question

Example:
Table: FC_GQ_SOURCE_ANALYSIS
Sample_type = C
Effective_date = null
Sample_date = 20/13/07/14 09:00:00
What needs to happen is for any sample analysis that has a sample_type of "C" we need the effective date to be the first of the month for whatever month the sample date is and keep the time as it is…
so in the example data the effective date should be 2013/07/01 09:00:00.
How would I update just the day part of that date string?

Answer 1

Hi,
You don't need to convert to a string and back again to do that.  Use TRUNC to get the right day, and date arithmetic to get the right time, like this:
UPDATE  fc_gq_source_analysis
SET     effective_date = TRUNC (sample_date, 'MONTH')
                             + (sample_date - TRUNC (sample_date))
WHERE   sample_type  = 'C'
TRUNC (sample_date, 'MONTH')        is midnight on the 1st of the month that you want.
sample_date - TRUNC (sample_date)   is the time that you want, expressed as a number of days (e.g., 9/24 of a day).