Question

I’m SELECTing some aggregate data and grouping on the date and a particular field. I want to display all values in that field and a count for those values even if there was no data matching that field on that day. E.g.

Date        MyField  Count
2009-09-25  A        2
2009-09-25  B        0
2009-09-24  A        1
2009-09-24  B        1

The Oracle SQL I currently have to do this is akin to the following:

SELECT today,
       mytable.myfield,
       COUNT(
         CASE WHEN fields.myfield = mytable.myfield AND
                   date >= today AND
                   date < tomorrow
              THEN 1
         END
       )
FROM (
       SELECT TRUNC(SYSDATE) + 1 - LEVEL AS today,
              TRUNC(SYSDATE) + 2 - LEVEL AS tomorrow
       FROM DUAL
       CONNECT BY LEVEL <= 30
     ),
     (
       /* This is the part that seems inefficient */
       SELECT DISTINCT myfield
       FROM mytable
       WHERE myfield IN ('A', 'B')
     ) fields,
     mytable
GROUP BY today, mytable.myfield
ORDER BY today DESC, mytable.myfield ASC

My concern is that I know exactly which values I want to display for myfield, and it seems inefficient to have a SELECT query that accesses mytable. I was wondering if there’s some way I could do something like this in that sub-query:

SELECT ('A', 'B') AS myfield
FROM DUAL

I’m using an older version of Oracle where WITH clauses do not work.

Answer 1

You would have to get them as different rows, not different columns. So you’ll end up with

select 'A' from dual
union
select 'B' from dual

In that case, the query should be equivalent as long as there are rows in mytable with fields ‘A’ and ‘B’. If ever there aren’t, then your subquery will return rows that the original subquery would not.