NULL is a value for an attribute value that contains no data. Sometimes not allowed due to 'not null constraints' or slt.
NULL is a value for an attribute value that contains no data. NULL is not the same as a blank in a field or a zero value. When a record is created a NULL value is where it has been left blank.
Sometimes not allowed due to 'not null constraints' or slt.
When querying for NULL Values be aware not to use val=NULL or val!=NULL but val IS (NOT) NULL or (NOT) val IS NULL.
cp. this question: What is the difference between != NULL and IS NOT NULL?
