Oracle: select missing dates

Question

I have a table with (among other things) dates in a field.

I need to get a list of all dates that are more recent than the oldest date, older than the most recent date, and are completely missing from the table.

So, if the table contained:

2012-01-02
2012-01-02
2012-01-03
2012-01-05
2012-01-05
2012-01-07
2012-01-08

I want a query that returns:

2012-01-04
2012-01-06

Answer 1

Something like this (assuming your table is named your_table and the date column is named the_date):

with date_range as (
      select min(the_date) as oldest, 
             max(the_date) as recent, 
             max(the_date) - min(the_date) as total_days
      from your_table
),
all_dates as (
   select oldest + level - 1 as a_date
   from date_range
   connect by level <= (select total_days from date_range)
)
select ad.a_date
from all_dates ad
  left join your_table yt on ad.a_date = yt.the_date
where yt.the_date is null
order by ad.a_date;  

Edit:
the WITH clause is called a "common table expression" and is equivalent to a derived table ("inline view").

It's similar to

select * 
from ( 
     ..... 
) all_dates
join your_table ...

The second CTE simply creates a list of dates "on-the-fly" using a undocumented feature of Oracle's connect by implementation.

Re-using a select (like I did with calculating the first and last date) is a bit easier (and IMHO more readable) than using derived tables.

Edit 2:

This can be done with a recursive CTE as well:

with date_range as (
      select min(the_date) as oldest, 
             max(the_date) as recent, 
             max(the_date) - min(the_date) as total_days
      from your_table
),
all_dates (a_date, lvl) as (
   select oldest as a_date, 1 as lvl
   from date_range 
   union all
   select (select oldest from date_range) + lvl, lvl + 1
   from all_dates 
   where lvl < (select total_days from date_range)
)
select ad.a_date, lvl
from all_dates ad    
  left join your_table yt on ad.a_date = yt.the_date
where yt.the_date is null
order by ad.a_date;  

Which should work in all DBMS supporting recursive CTEs (PostgreSQL and Firebird - being more standard compliant - do need the recursive keyword though).

Note the hack select (select oldest from date_range) + lvl, lvl + 1 in the recursive part. This should not be necessary, but Oracle still has some bugs with regards to DATEs in a recursive CTE. In PostgreSQL the following works without problems:

....
all_dates (a_date, lvl) as (
   select oldest as a_date, 0 as lvl
   from date_range 
   union all
   select a_date + 1, lvl + 1
   from all_dates 
   where lvl < (select total_days from date_range)
)
....