Change result set order based on another column being null - sql

Question

I have a query that sorts the results based on the second column.
select decode(name, null, 'n/a', name) name, value1
from tableA
group by name
order by 2
How can I change it so a result where the first column is null is always the last row in the result set, without changing the ordering for the rest of the results?
One solution I have in my mind is to use union two queries, one which excludes nulls and one which only has nulls, something like:
select decode(name, null, 'n/a', name) name, value1
from tableA
where name is not null
group by name
union
select decode(name, null, 'n/a', name) name, value1
from tableA
where name is null
group by name
order by 2
Is there a better way to do this?

Answer 1

Your union approach won't get the result you want; the order by is applied after the two queries are unioned, so they will be sorted the same as if you had a single query. You could add a flag column in each brach of the union and include that in the ordering, but the you need to exclude it from the final select list.
You can handle this in the order by clause, using a case statement (or decode if you prefer) against column 1 which treats all not-null values as one priority regardless of the actual value, and all null values as a different priority; and then further orders by column 2:
select decode(a.name, null, 'n/a', a.name) name, value1
from tableA a
order by case when a.name is null then 1 else 0 end, value1 desc
I've used a table alias and included that in the case to avoid confusion between the original table value and the column alias of the same name. This will put all the results with null ('n/a') names after all of those which are not null; and within each category all the results will still be sorted by the second column.
With some sample data:
with tableA (name, value1) as (
select 'Joe', 3 from dual
union all select 'Anne', 10 from dual
union all select null, 4 from dual
union all select 'Sarah', 2 from dual
union all select 'Bill', 5 from dual
union all select 'Mary', 7 from dual
)
... ordering just by the second column gets:
select decode(a.name, null, 'n/a', a.name) name, value1
from tableA a
order by value1 desc;
NAME VALUE1
----- ----------
Anne 10
Mary 7
Bill 5
n/a 4
Joe 3
Sarah 2
Adding this case clause puts your 'n/a' row last in the result set:
select decode(a.name, null, 'n/a', a.name) name, value1
from tableA a
order by case when a.name is null then 1 else 0 end, value1 desc;
NAME VALUE1
----- ----------
Anne 10
Mary 7
Bill 5
Joe 3
Sarah 2
n/a 4
(I've ignored the group by clause in your example because you don't have any aggregates, and neither of your queries is valid; but you can still group and do this in your real query if you need to.)