Date Range Overlap with Nullable Dates

Question

I\'m looking for an extended answer to the question asked here:

Determine Whether Two Date Ranges Overlap

where any of the dates in either date range can be null

Answer 1

This case can be handled by a slight generalization of Charles Bretana's excellent answer to that question.

Let CondA Mean DateRange A Completely After DateRange B (True if StartA > EndB) Let CondB Mean DateRange A Completely Before DateRange B (True if EndA < StartB)

In this case, assuming you want a null date to represent "no starting/ending bound," the conditions are modified. For CondA, for instance, in order for DateRange A to be completely after DateRange B, DateRange A must have a defined starting time, DateRange B must have a defined ending time, and the starting time of A must be after the ending time of B:

CondA := (StartA != null) && (EndB != null) && (StartA > EndB)

CondB is the same with A and B switched:

CondB := (StartB != null) && (EndA != null) && (StartB > EndA)

Continuing,

Then Overlap exists if Neither A Nor B is true

Overlap := !(CondA || CondB)

and

Now deMorgan's law, I think it is, says that

Not (A Or B) <=> Not A And Not B

Overlap == !CondA && !CondB
        == ![(StartA != null) && (EndB != null) && (StartA > EndB)] &&
           ![(StartB != null) && (EndA != null) && (StartB > EndA)]
        == [(StartA == null) || (EndB == null) || (StartA <= EndB)] &&
           [(StartB == null) || (EndA == null) || (StartB <= EndA)]

I think this is actually a bit more robust than the solution you developed, because if EndB == NULL but StartA is not null, your first condition will wind up comparing StartA <= NULL. In most languages I'm familiar with, that's an error condition.