Empty string vs NULL

Some differences between them:

• NULL can be assigned to any type, as opposed to empty string which won't be compatible with date/numerical fields.
• NULL is an UNKNOWN value, it doesn't have a value as opposed to an empty string, which is a value, but empty one.
• As far as I'm aware of, NULL shouldn't capture memory as opposed to an empty string which does.
• null = null will result in null as opposed to ''='' which will result in TRUE.

You can have your query modified as below:

select * from zzz_fkp_registration_female where isnull(fname,'') = '';

This query will result all the blanks cells as well as cell with null values.

Note:

1. NULL values represent missing unknown data.
2. Blank data is actual data entered as blank during input.