Question

I have a table that is to be sorted based on two columns of numbers (call them Column X and Column Y). In each row, the two columns can either both have numeric values (in which case X<=Y), or one of the columns can be NULL.

Example data:

      X     Y
    ----- -----
... NULL   26
...  31   NULL
...   1     7
...  39    46
... NULL   36
...  15    16
... NULL   14
...  23    29

I would like to sort this data so that the columns interleave “correctly”. Specifically:

1) If the X value is present in both rows, order based on X.

2) Else, if the Y value is present in both rows, order based on Y.

3) Else, compare the available X value and the available Y value.

The “correct” sorting of the example data would be:

      X     Y
    ----- -----
...   1     7
... NULL   14
...  15    16
... NULL   26
...  23    29
...  31   NULL
... NULL   36
...  39    46

Is there any simple way to perform this sorting, in an ORDER BY clause?

Answer 1

You can’t. The order is not well defined

The simple set

5    10
7    null
null 8

can be sorted

null 8
5    10
7    null

and

5    10
7    null
null 8

depending on where you start sorting.

If possible I would change the sort criteria to “X if available, otherwise Y“. Then you could use the COALSECE operator as suggested by “mu is too short”. (order by coalesce(x, y))