将相关子查询转换为JOIN

Question

I have a correlated subquery which is creating performance issues. Being a correlated subquery it doesn't accept index hints either. So I am trying to convert it into a JOIN instead. Please find below the original query and the modified query. The modified query doesn't return any row but the original returns me 224 rows.

Appreciate any insight on what is wrong with my JOIN query, if it even makes sense to use JOIN instead of subquery in this case. Thanks.

select Area_CODE,
       due_DATE,
       RATE,

from SCHED S
where (s.Area_CODE = 11001 and 
   (s.COMP_CODE = 'a' 
   or
      (s.COMPANY_CODE = 'b' 
          and s.due_DATE <
          (
           select
               nvl( min(s1.due_DATE), to_date  ( '31-DEC-2999', 'DD-MM-YYYY') )
               from   SCHED s1
               where  s1.AREA_CODE = s.AREA_CODE 
               and s1.COMP_CODE = 'c'
          )
       )
    )
   )
order by a.EFF_DATE asc, s.due_DATE asc

Modified Query:

SELECT
  Area_CODE,
       due_DATE,
       RATE

from SCHED S
LEFT JOIN
  (
    SELECT
      NVL( MIN(s1.due_DATE), to_date ( '31-DEC-2999', 'DD-MM-YYYY') ) AS
      min_date,
      s1.AREA_CODE AS a_code
    FROM
      SCHED s1
    WHERE
      s1.COMPANY_CODE = 'c'
    GROUP BY
      s1.AREA_code
  )
  s2
ON
  s2.A_CODE = s.area_code
WHERE
  (
    s.area_code = 11001
  AND
    (
      s.COMP_CODE = 'a'
    OR
      (
        s.COMP_CODE = 'b' 
      and s.due_DATE < s2.min_date
      )
    )
  )
order by s.EFF_DATE asc, s.due_DATE asc

Answer 1

The difference is where there are no dates for a given area code

Change the left join (s2) to an inner join on

inner JOIN 
( 
    SELECT 
        min(s1.due_DATE) AS min_date, 
        s1.AREA_CODE AS a_code 
    FROM 
        (
            select Area_Code, due_date from SCHED WHERE COMPANY_CODE = 'c' 
            union
            select Area_Code, to_date('31-dec-2999','DD-MM-YYYY') from SCHED
        ) s1    
    GROUP BY 
        s1.AREA_code 
) s2