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Handling NULL sets within the SQL CASE statement

Oracle Database Tips by Donald BurlesonNovember 7, 2015


Question:  I'm trying to write a case statement that will query the maximum date from a table, and if the result is null- return sysdate. 

select max(case somedate when NULL then sysdate
else somedate
end) somedate
from trydates
where somedate < (sysdate-4)

will return max(somedate) even if the condition returns no results (when the condition is: somedate < sysdate-40, there is no matching value and I get ' ' as a result). Is there a way I can catch that NULL returned when there is no date returned by the condition at the end?  My original statement was using decode function:

   NULL, sysdate, MAX(somedate))
FROM trydates
   somedate BETWEEN :lower_active_bound AND :from_date


Answer:  Oracle CASE SQL allows you to add "Boolean logic" and branching using the decode and CASE clauses. The case statement is a more flexible extension of the Decode statement. In its simplest form the Oracle CASE function is used to return a value when a match is found, it it's trickier to handle a NULL condition from a subquery.

Laurent Schneider provides this answer:

select max(case somedate when NULL then sysdate
else somedate
end) somedate
from trydates
where somedate < (sysdate-4)
         case when somedate when NULL then sysdate
         else somedate
      end) IS NOT NULL


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