ORACLENERD: Fun with SQL

Fun with SQL

My group held it's first Peer Code Review today. It was my code (the code I've screwed up a couple of times). One of the requirements (I stuck to them this time, yeah for me!) was to limit the data we were pulling from another system to only the last 48 months. There were three different procedures that needed this date but I didn't want to break the existing automated job so I defaulted everything to:

p_date_from DATE DEFAULT ADD_MONTHS( TO_DATE( TO_CHAR( SYSDATE, 'YYYYMM' ), 'YYYYMM' ), -48 )

I wanted the last full 48 months and this worked just fine.

This is a fairly long process which takes anywhere from 8 to 12 hours to run, depending on system resources. One of my colleagues asked what would happen in the event of a crash (on the last day of the month) and the required re-start (on the first day of the month). One set of data would have everything going back 49 months and the other 48 months which would create a lot of errors. A great catch.

Since we're in maintenance mode now (re-architecting the whole thing), we decided not to stop the deployment, we looked at the calendar to see when the first occurence was in which the 1st fell on a Sunday (the job is scheduled weekly on Sunday)...which now that I think about it, was probably wrong. We should have been looking for a Sunday in which was also the last day of the month. Anyway...

I like trying to answer questions with SQL. So I wanted to find the occurences on which Sunday was also the first day of the month (despite while writing this realizing that we should have been looking for the last day, again, digression). Here's my resultant query:


SELECT
  ADD_MONTHS( TO_DATE( TO_CHAR( SYSDATE, 'YYYYMM' ), 'YYYYMM' ), myrownum )first_day_on_sunday
FROM
  ( 
    SELECT rownum myrownum
    FROM dual
    CONNECT BY LEVEL < 100
  )
WHERE TO_CHAR( ADD_MONTHS( TO_DATE( TO_CHAR( SYSDATE, 'YYYYMM' ), 'YYYYMM' ), myrownum ), 'D' ) = 1
  AND TO_CHAR( ADD_MONTHS( TO_DATE( TO_CHAR( SYSDATE, 'YYYYMM' ), 'YYYYMM' ), myrownum ), 'DD' ) = 1
/

FIRST_DAY_
----------
06/01/2008
02/01/2009
03/01/2009
11/01/2009
08/01/2010
05/01/2011
01/01/2012
04/01/2012
07/01/2012
09/01/2013
12/01/2013
06/01/2014
02/01/2015
03/01/2015
11/01/2015