SELECT statement not returning NULL records

Question

I have the following SELECT statement:
SELECT *
FROM RPT_DS1_CNT_CAT
WHERE DUPS_SAME <> 'N/A';
I also tried:
SELECT *
FROM RPT_DS1_CNT_CAT
WHERE DUPS_SAME != 'N/A';
Same results - There ARE Null DUPS_SAME values
which are not being selected. I get NO results.
When I execute this, I do get NULL values.
SELECT *
FROM RPT_DS1_CNT_CAT
WHERE DUPS_SAME IS NULL;
How can I ensure that I get all non - 'N/A' records
if it won't pick up the nulls? Is there a function
I can use or a different wording? I do want to use
!= or <> because there may be other values besides
'N/A' such as 'YES' or 'NO' and they may be null or
filled with spaces.

Answer 1

The expression
NVL(DUPS_SAME,'') is meaningless. You're saying "If DUPS_SAME is NULL, return the empty string, which is NULL, otherwise return DUPS_SAME." If you're going to use NVL, the string that gets returned if the column is NULL should be non-NULL, i.e.
NVL(DUPS_SAME,'DUPS_SAME was NULL') Your query should, as others have pointed out, either be
SELECT *
  FROM rpt_ds1_cnt_cat
WHERE dups_same <> 'N/A' OR dups_same IS NULLor
SELECT *
  FROM rpt_ds1_cnt_cat
WHERE NVL(dups_same, 'DUPS_SAME is NULL') <> 'N/A' Either of these return the same results as your query but they're going to be far clearer for whoever needs to maintain the code. There may be performance differences as well if DUPS_SAME is indexed depending on the data distribution...
Justin