Difference between the queries - help needed.

Question

I have three tables Plan_Zip,Service_Zip,Plan_Info
Plan_Zip is the parent table and referenced by Service_Zip
and Service_Zip is referenced by Plan_Info.
I have this query:
select
     sz.ts_nbr,pz.packages_qty,pz.stops_qty
from
     plan_zip pz, service_zip sz, plan_info pi
where
     pi.plan_run_nbr = sz.plan_run_nbr and
     sz.service_run_nbr = pz.service_run_nbr and     
     pi.day_plan_nbr = 43027 and
     pi.run_type_nbr = 0 and
     sz.ts_nbr = 1 or sz.ts_nbr = 2
When I am running the query I am getting so many rows which are identical,
but when I modified the above query as:
select
     sz.ts_nbr,pz.packages_qty,pz.stops_qty
from
     plan_zip pz, service_zip sz, plan_info pi
where
     pi.plan_run_nbr = sz.plan_run_nbr and
     sz.service_run_nbr = pz.service_run_nbr and     
     pi.day_plan_nbr = 43027 and
     pi.run_type_nbr = 0 and
     (sz.ts_nbr = 1 or sz.ts_nbr = 2)
I am getting the correct results.
Please explain me the difference between the above two queries.
Regards
Raghu

Answer 1

Look at the below example
SQL> create table t
  2  as
  3  select level no, level no1, 0 no2
  4    from dual
  5  connect by level <= 5
  6  /
Table created.
SQL> select * from t
  2  /
        NO        NO1        NO2
         1          1          0
         2          2          0
         3          3          0
         4          4          0
         5          5          0
SQL> select *
  2    from t
  3   where no = 1
  4     and no1 = 1
  5      or no2 = 0
  6  /
        NO        NO1        NO2
         1          1          0
         2          2          0
         3          3          0
         4          4          0
         5          5          0Here what the where condition is telling is no should be 1, no1 should be 1 or no2 is 0. you can read it like this.
if (no = 1 and no1 = 1) or (no2 = 0)
then
     true ;
else
     false;
end if;so what i does is no2 = 0 is satisfied by all the rows and hence all the rows are returned.
Next...
SQL> select *
  2    from t
  3   where no = 1
  4     and (no1 = 1 or no2 = 0)
  5  /
        NO        NO1        NO2
         1          1          0This can be read like this.
if (no = 1) and (no1 = 1 or no2 = 0)
then
     true ;
else
     false;
end if;Here no = 1 is a must satisfy condition. Once that is satisfied then no1 can be 1 or no2 can be 0 which ever is
satisfied consider that. So the first condition no=1 is satisfied by only one record and hence that alone is returned.
Thanks,
Karthick.