Question

If I have a table with three columns

userid,eventid,entrytime
 

and values like

userid,eventid,entrytime
  1      1      NULL
  2      1    2012-01-01
  1      2    2012-01-01
  2      2    2012-01-01
  3      1      NULL
  3      2      NULL

What is the best way of selecting userids where entrytime has always been NULL

Answer 1

Since a COUNT(entrytime) aggregate will eliminate NULL values, you can use it to determine which userid has no non-null value for entrytime by comparing it to 0 in the HAVING clause.

SELECT userid, COUNT(entrytime)
FROM yourtable
GROUP BY userid
HAVING COUNT(entrytime) = 0

Here is a live demonstration (Result is userid = 3)