What is in the column, empty string, null or ....?

Question

I am using Oracle 11.2, have one table tb_class as following:
TB_CLASS (id number(3), nm varchar2(32), seqNum number(3));
1, ,212
2,'as', 12
3, 'qq', 12
select * from TB_CLASS where id=1 and nm=null; ...no row selected
select nvl(nm) from TB_CLASS where id=1; ...1 row selected
Any one know what is in column nm for row id=1? I thought it is empty string, but why it returns no row in the first query?
Thanks!

Answer 1

1) In Oracle there is no difference between NULL and the empty string.
2) NULL is never equal to anything (including another NULL). NULL is never unequal to anything (including another NULL). You need to use the IS NULL and/ or IS NOT NULL operators in order to look for NULL values
SELECT *
  FROM tb_class
WHERE nm IS NULLor
SELECT *
  FROM tb_class
WHERE nm IS NOT NULLOf course, it is also possible that you are stating that the NM column contains one or more spaces or that it contains one or more non-printable characters rather than a NULL. That's why you were asked what dump(nm) returns
Justin