Select Row immediately previous and following a date range

Question

I have a query which returns two values within a given date range (colA, colB). This works great. However, I also need to return (in a separate query) the entries immediately previous and following this current date range (same two values).
I am more familiar with MySQL where the query would include an "Order by date desc limit 1" for the row previous to the original query or "Order by date asc limit 1" for the row following the original query.
What is the best way to return these rows within Oracle?

Answer 1

Hi,
This is an example of a Top-N Query, where you want to pick N items (N=1 in this case) from the top of an ordered list.
This problem is a little trickier, because you want to pick from two lists, the rows before the specieifed date range (in descending order) and the rows after the range.
The easiest way might be a UNION of two separate queries, using WHERE ROWNUM = 1 in each one.
The solution below is easier to generalize, e.g. if you want the first and last row from each department.
WITH
e     AS
     SELECT     empno, ename, hiredate
     ,     ROW_NUMBER () OVER
          ( ORDER BY     CASE
                         WHEN hiredate < TO_DATE ( '01-JAN-1982'
                                        , 'DD-MON-YYYY'
                                        ) -- Start of date range
                         THEN hiredate
                    END
                    DESC     NULLS LAST
          ) AS pre_num
     ,     ROW_NUMBER () OVER
          ( ORDER BY     CASE
                         WHEN hiredate > TO_DATE     ( '31-DEC-1982'
                                        , 'DD-MON-YYYY'
                                        ) -- End of date range
                         THEN hiredate
                    END
          ) AS post_num
     FROM     scott.emp
SELECT     empno, ename, hiredate
FROM     e
WHERE     pre_num          = 1
OR     post_num     = 1
;Results (last hire before 1982 and first hire after 1982):
     EMPNO ENAME      HIREDATE
      7788 SCOTT      19-APR-87
      7900 JAMES      03-DEC-81