Wrong colum definition? or other reasons?

Question

Table 1
Col 1 : Flag1
one:
two:
Flag1 will be set manually based on some conditions. Currently, none of the records in the db mets the condition, hence the field is empty. Users will log into the db and update the Flag1 to Y for those records that meet the conditions. Once a month, i will run the query below to extract those that does not have 'y' in the Flag1 column
select a.rowid,a.* from table1 a where a.flag1 <> 'y' */The above code returns nothing.
select a.rowid, a.* from table1 a The above code returns all rows.
Col 1 : Flag1
one:
two:
why the the first query come back with nothing? Shouldn't it return all rows?

Answer 1

NULL in flag1 column makes the difference. That is why, good design should make flag1 NOT NULL and store either 'Y' or 'N' based on your business conditions.
SQL>drop table a
  2  /
Table dropped.
SQL>create table a (col1 varchar2(5), col2 varchar2(1))
  2  /
Table created.
SQL>insert into a values('one', NULL)
  2  /
1 row created.
SQL>insert into a values('two', NULL)
  2  /
1 row created.
SQL>commit
  2  /
Commit complete.
SQL>select a.rowid, a.* from a
  2  where a.col2 <> 'Y'
  3  /
no rows selected
SQL>c/a.col2/nvl(a.col2, 'N')
  2* where nvl(a.col2, 'N') <> 'Y'
SQL>;
  1  select a.rowid, a.* from a
  2* where nvl(a.col2, 'N') <> 'Y'
SQL>/
ROWID              COL1  C
AAANH1AALAAAAIiAAA one
AAANH1AALAAAAIiAAB two
SQL>Cheers
Sarma.