Query returning different counts

Question

Dear all,
10.2.0.4 on solaris ..
Why is the 2 queries are displaying different rows
SQL> select count(*) FROM MEDT_BL where rowid  in(SELECT rowid FROM MEDT_BL WHERE BILLDATE IN ('21-JUN-2011','21-MAY-2011','21-APR-2011') UNION ALL
SELECT rowid FROM MEDT_BL WHERE NVL(BILLED,'N') = 'Y');  2
  COUNT(*)
  40131732
SQL> select count(*) from (SELECT * FROM MEDT_BL WHERE BILLDATE IN
('21-JUN-2011','21-MAY-2011','21-APR-2011') UNION ALL SELECT * FROM MEDT_BL WHERE NVL(BILLED,'N') <> 'Y')  2
  COUNT(*)
   7426398Thanks
Kai

Answer 1

Your second query double counts any records that meet both conditions while you first query counts each row only once if it matches either condition. The sub-query in the IN clause is implicitly "distincted". Consider
SQL > select * from t;
        ID DESCR
         1 One
         2 Two
         3 Three
         4 Four
         5 Five
         6 Six
         7 Seven
         8 Eight
         9 Nine
        10 Ten
        11 Eleven
        12 Twelve
        13 Thirteen
        14 Fourteen
        15 Fifteen
SQL > select * from t
  2   where rowid in (select rowid from t
  3                   where mod(id, 2) = 0
  4                   union all
  5                   select rowid from t
  6                   where descr like '%teen');
        ID DESCR
         2 Two
         4 Four
         6 Six
         8 Eight
        10 Ten
        12 Twelve
        13 Thirteen
        14 Fourteen
        15 Fifteen
SQL > select *
  2   from (select * from t
  3         where mod(id, 2) = 0
  4         union all
  5         select * from t
  6        where descr like '%teen');
        ID DESCR
         2 Two
         4 Four
         6 Six
         8 Eight
        10 Ten
        12 Twelve
        14 Fourteen
        13 Thirteen
        14 Fourteen
        15 FifteenIn the first version the row for 14, Fourteen only appeared once because the rowids were "distincted" while in the second query, it appears twice. Once from the top half of the union because 14 is even, and once from the bottom half of the union because Fourteen ends with teen.
To make the second query equivalent to the first, you would need to use a union all, or combine both predicates into one query like:
select count(*)
from MEDT_BL
where billdate in ('21-JUN-2011','21-MAY-2011','21-APR-2011') or
      nvl(billed,'N') = 'Y')As a side note, '21-JUN-2011','21-MAY-2011', and '21-APR-2011' are not dates, they are strings. Assuming that the billdate column is actually a date datatype, you need to use to_date to make the comparision correct:
select count(*)
from MEDT_BL
where billdate in (TO_DATE('21-JUN-2011', 'dd-mon-yyyy'),
                   TO_DATE('21-MAY-2011', 'dd-mon-yyyy'),
                   TO_DATE('21-APR-2011', 'dd-mon-yyyy')) or
      nvl(billed,'N') = 'Y')John