SQL query of only specified rows

Ignition
#1

Hello all, I am having a little difficulty here. I have to run a daily automated report that pulls data from the previous 24 hour period. The DB records data every minute and I have the below instruction that works great. However, I now have to pull a separate report that only pulls the data for every 30 minutes. Is there a way to adjust this code to only pull the previous days data and only the rows that have the 30 minute data? I hope I am explaining this well enough. Thanks in advance for any assistance.

SELECT CEMS_DATA_MIN.DATE_TIME,
CEMS_DATA_MIN.UNIT_ID,
CEMS_DATA_MIN.DATA32,
CEMS_DATA_MIN.DATA30
FROM CEMS_DATA_MIN
WHERE CEMS_DATA_MIN.UNIT_ID = 2154 AND ROWNUM <= 1440
ORDER BY CEMS_DATA_MIN.DATE_TIME DESC

#2

What about adding

AND ROWNUM % 30 = 0

to the where line? Should get row 0, row 30, row 60, etc. And you want just that one line, not some sort of aggregate, right?

#3

I do actually have to generate a min, max, and average at the bottom of the report.