Oracle SQL (Toad): Expand table - sql

Question

Suppose I have an SQL (Oracle Toad) table named "test", which has the following fields and entries (dates are in dd/mm/yyyy format):
id ref_date value
---------------------
1 01/01/2014 20
1 01/02/2014 25
1 01/06/2014 3
1 01/09/2014 6
2 01/04/2015 7
2 01/08/2015 43
2 01/09/2015 85
2 01/12/2015 4
I know from how the table has been created that, since there are value entries for id = 1 for February 2014 and June 2014, the values for March through May 2014 must be 0. The same applies to July and August 2014 for id = 1, and for May through July 2015 and October through November 2015 for id = 2.
Now, if I want to calculate, say, the median of the value column for a given id, I will not arrive at the correct result using the table as it stands - as I'm missing 5 zero entries for each id.
I would therefore like to create/use the following (potentially just temporary table)...
id ref_date value
---------------------
1 01/01/2014 20
1 01/02/2014 25
1 01/03/2014 0
1 01/04/2014 0
1 01/05/2014 0
1 01/06/2014 3
1 01/07/2014 0
1 01/08/2014 0
1 01/09/2014 6
2 01/04/2015 7
2 01/05/2015 0
2 01/06/2015 0
2 01/07/2015 0
2 01/08/2015 43
2 01/09/2015 85
2 01/10/2015 0
2 01/11/2015 0
2 01/12/2015 4
...on which I could then compute the median by id:
select id, median(value) as med_value from test group by id
How do I do this? Or would there be an alternative way?
Many thanks,
Mr Clueless

Answer 1

In this solution, I build a table with all the "needed dates" and value of 0 for all of them. Then, instead of a join, I do a union all, group by id and ref_date and ADD the values in each group. If the date had a row with a value in the original table, then that's the resulting value; and if it didn't, the value will be 0. This avoids a join. In almost all cases a union all + aggregate will be faster (sometimes much faster) than a join.
I added more input data for more thorough testing. In your original question, you have two id's, and for both of them you have four positive values. You are missing five values in each case, so there will be five zeros (0) which means the median is 0 in both cases. For id=3 (which I added) I have three positive values and three zeros; the median is half of the smallest positive number. For id=4 I have just one value, which then should be the median as well.
The solution includes, in particular, an answer to your specific question - how to create the temporary table (which most likely doesn't need to be a temporary table at all, but an inline view). With factored subqueries (in the WITH clause), the optimizer decides if to treat them as temporary tables or inline views; you can see what the optimizer decided if you look at the Explain Plan.
with
inputs ( id, ref_date, value ) as (
select 1, to_date('01/01/2014', 'dd/mm/yyyy'), 20 from dual union all
select 1, to_date('01/02/2014', 'dd/mm/yyyy'), 25 from dual union all
select 1, to_date('01/06/2014', 'dd/mm/yyyy'), 3 from dual union all
select 1, to_date('01/09/2014', 'dd/mm/yyyy'), 6 from dual union all
select 2, to_date('01/04/2015', 'dd/mm/yyyy'), 7 from dual union all
select 2, to_date('01/08/2015', 'dd/mm/yyyy'), 43 from dual union all
select 2, to_date('01/09/2015', 'dd/mm/yyyy'), 85 from dual union all
select 2, to_date('01/12/2015', 'dd/mm/yyyy'), 4 from dual union all
select 3, to_date('01/01/2016', 'dd/mm/yyyy'), 12 from dual union all
select 3, to_date('01/03/2016', 'dd/mm/yyyy'), 23 from dual union all
select 3, to_date('01/06/2016', 'dd/mm/yyyy'), 2 from dual union all
select 4, to_date('01/11/2014', 'dd/mm/yyyy'), 9 from dual
),
-- the "inputs" table constructed above is for testing only,
-- it is not part of the solution.
ranges ( id, min_date, max_date ) as (
select id, min(ref_date), max(ref_date)
from inputs
group by id
),
prep ( id, ref_date, value ) as (
select id, add_months(min_date, level - 1), 0
from ranges
connect by level <= 1 + months_between( max_date, min_date )
and prior id = id
and prior sys_guid() is not null
),
v ( id, ref_date, value ) as (
select id, ref_date, sum(value)
from ( select id, ref_date, value from prep union all
select id, ref_date, value from inputs
)
group by id, ref_date
)
select id, median(value) as median_value
from v
group by id
order by id -- ORDER BY is optional
;
ID MEDIAN_VALUE
-- ------------
1 0
2 0
3 1
4 9

Answer 2

If ref_date is date and is second
with int1 as (select id
, max(ref_date) as max_date
, min(ref_date) as min_date from test group by id )
, s(n) as (select level -1 from dual connect by level <= (select max(months_between(max_date, min_date)) from int1 ) )
select i.id
, add_months(i.min_date,s.n) as ref_date
, nvl(value,0) as value
from int1 i
join s on add_months(i.min_date,s.n) <= i.max_date
LEFT join test t on t.id = i.id and add_months(i.min_date,s.n) = t.ref_date
And with median
with int1 as (select id
, max(ref_date) as max_date
, min(ref_date) as min_date from test group by id )
, s(n) as (select level -1 from dual connect by level <= (select max(months_between(max_date, min_date)) from int1 ) )
select i.id
, MEDIAN(nvl(value,0)) as value
from int1 i
join s on add_months(i.min_date,s.n) <= i.max_date
LEFT join test t on t.id = i.id and add_months(i.min_date,s.n) = t.ref_date
group by i.id