Select default in case of no value returned - oracle

Question

I am trying to get some default value in my resultset if query does not return anything. I am trying nvl for the same but it is not returning the expected default value. To simulate, Consider following query,
select nvl(null, '10') from dual where 1=0;
I want to get 10 in case of given condition is not true and query does not return any value. However above query not returning any row.

Answer 1

Your query returns zero rows. NVL() isn't going to change that (*).
The correct solution is for the program which executes the query to handle NO_DATA_FOUND exception rather than fiddling the query.
However, you need a workaround so here is one using two sub-queries, one for your actual query, one to for the default.
When your_query returns an empty set you get this:
SQL> with your_qry as
2 ( select col1 from t42 where 1=0 )
3 , dflt as
4 ( select 10 as col1 from dual )
5 select col1
6 from your_qry
7 union all
8 select col1
9 from dflt
10 where not exists (select * from your_qry );
COL1
----------
10
SQL>
And when it returns a row you get this:
SQL> with your_qry as
2 ( select col1 from t42 )
3 , dflt as
4 ( select 10 as col1 from dual )
5 select col1
6 from your_qry
7 union all
8 select col1
9 from dflt
10 where not exists (select * from your_qry );
COL1
----------
12
13
SQL>
The WITH clause is optional here, it just makes it easier to write the query without duplication. This would have the same outcome:
select col1
from t42
where col0 is null
union all
select 10
from dual
where not exists (select col1
from t42
where col0 is null)
;
(*) Okay, there are solutions which use NVL() or COALESCE() with aggregations to do this. They work with single column projections in a single row as this question poses, but break down when the real query has more than one row and/or more than one column. Aggregations change the results.
So this looks alright ...
SQL> with cte as (
2 select 'Z' as col0, 12 as col1 from dual where 1=0 union all
3 select 'X' as col0, 13 as col1 from dual where 1=0 )
4 select
5 nvl(max(col0), 'Y') as col0, nvl(max( col1), 10) as col1
6 from cte;
COL0 COL1
---------- ----------
Y 10
SQL>
... but this not so much:
SQL> with cte as (
2 select 'Z' as col0, 12 as col1 from dual union all
3 select 'X' as col0, 13 as col1 from dual )
4 select
5 nvl(max(col0), 'Y') as col0, nvl(max( col1), 10) as col1
6 from cte;
COL0 COL1
---------- ----------
Z 13
SQL>

Answer 2

May be something like this is what you need
You could change WHERE clause (in this case WHERE COL > 1) similarly in both places.
WITH T(COL) AS(
SELECT 1 FROM DUAL UNION ALL
SELECT 2 FROM DUAL UNION ALL
SELECT 3 FROM DUAL
)
SELECT COL FROM T WHERE COL > 1
UNION ALL
SELECT 10 AS COL FROM DUAL WHERE NOT EXISTS( SELECT 1 FROM T WHERE COL > 1)

Answer 3

You can use aggregation. An aggregation query always returns one row:
select coalesce(max(null), '10')
from dual
where 1 = 0;
I prefer coalesce() to nvl() because coalesce() is the ANSI standard function. But, nvl() would work here just as well.