Oracle OR MySQL - Custom ORDER with different criterea depending on NULL values - mysql

Question

I have a table with two pairs of DATE. I want to order the result based on the time of these dates. Each row have at least one pair of dates (when a row has both intervals: start2 > start1, end2 > end1 and start2 >= end1), like this:
| start1 | end1 | start2 | end2
row 1 | 4:00 | 5:00 | 5:00 | 6:00
row 2 | 4:30 | 5:00 | NULL | NULL
row 3 | NULL | NULL | 5:30 | 6:00
row 4 | 5:00 | 6:00 | 6:00 | 7:00
When two rows have both pairs, they should be compared by the start1.
When one row have only pair1 and the other have only pair2, start1 should be compared to start2
When one row have only one pair (any) and the other have both pairs, they should be compared by the pair that the first row have (start1 to start1 or start2 to start2). E.g.: if the first row has only the start2 and end2, and the second row has start1, end1, start2 and end2, these two rows should be compared by start2 only (start1 from the second row should be ignored)
How can I accomplish that?
EDIT
I can easily do that in C#, but I need to do this in database. Below the code of how it should work in C#:
static void Main(string[] args)
{
...
intervals.Sort(new IntervalComparer());
}
public class IntervalComparer : IComparer<Interval>
{
public int Compare(Interval quadro1, Interval quadro2)
{
int result = 0;
if (quadro1.start1 != null && quadro2.start1 != null)
result = quadro1.start1.Value.CompareTo(quadro2.start1.Value);
else if (quadro1.start2 != null && quadro2.start2 != null)
result = quadro1.start2.Value.CompareTo(quadro2.start2.Value);
else if (quadro1.start1 != null)
result = quadro1.start1.Value.CompareTo(quadro2.start2.Value);
else
result = quadro1.start2.Value.CompareTo(quadro2.start1.Value);
return result;
}
}

Answer 1

There is a flaw in your logic. Consider the following example:
| start1 | start2
a | 4:00 | 7:00
b | 5:00 | NULL
c | NULL | 6:00
For that data the following would be true using your logic:
a < b (a.start1 = 4:00 < b.start1 = 5:00)
b < c (b.start1 = 5:00 < c.start2 = 6:00)
c < a (c.start2 = 6:00 < a.start2 = 7:00)
And thus a < b < c < a is also "true". But if you want a meaningfull order, the < oprator should be transitive. That means if a < b is TRUE and b < c is TRUE, a < c should also be TRUE. But this is not the case.
So you could order the three rows as [a, b, c] or [b, c, a] or [c, a, b]. For me it doesn't make sense.
If you want an SQL solution, you should redefine the logic. I suggest to fill the NULLs using an estimated (average) difference (AVG(start2 - start1)). In my example the average difference would be 3 hours. So b.start2 would be replaced with b.start1 + 3 hours = 8:00. c.start1 would be replaced with c.start2 - 3 hours = 3:00. You could now order by the estimated values.
MySQL example:
select t.*
, d.avg_diff
, coalesce(time_to_sec(t.start1), time_to_sec(t.start2) - d.avg_diff) as estimated1
, coalesce(time_to_sec(t.start2), time_to_sec(t.start1) + d.avg_diff) as estimated2
from my_table t
cross join (
select avg(time_to_sec(start2) - time_to_sec(start1))
as avg_diff
from my_table
) d
order by estimated1, estimated2;
You can of course use the estimation expressions in the ORDER BY clause:
select t.*
from my_table t
cross join (
select avg(time_to_sec(start2) - time_to_sec(start1))
as avg_diff
from my_table
) d
order by
coalesce(time_to_sec(t.start1), time_to_sec(t.start2) - d.avg_diff),
coalesce(time_to_sec(t.start2), time_to_sec(t.start1) + d.avg_diff);
Demo: http://rextester.com/HHLT51865
Demo with original data: http://rextester.com/AMJIDT94457

Answer 2

try using the IF function, it should be something along the lines of:
SELECT if(start1 not is null, TIME(start1), TIME(start2) ) AS sortable_value
FROM your_table ORDER by sortable_value
Your whole logic can be simplified as in the above query. It basically gets reduced to if start1 is not null then sort by its time, else sort by start2's time.
If I misinterpreted something then you can add another IF instead of TIME(start1) (which is equivalent of the then branch) or instead of TIME(start2) (equivalent of the else branch).