Definition - What does Null mean?
Null, in a database context, is the total absence of a value in a certain field and means that the field value is unknown. Null is not the same as a zero value for a numerical field, text field or space value. Null implies that a database field value has not been stored.
Techopedia explains Null
A null cannot be compared to a value. For example, if a query is directed to a Customer_Addresses table to retrieve all customers without email addresses, then the Structured Query Language (SQL) query cannot be written as follows: SELECT * FROM Customer_ Addresses WHERE Email_Address=null. Instead, so as not to introduce a comparison with a null, the query must be written as follows: SELECT * FROM Customer_ Addresses WHERE Email_Address IS null.
When the values in a column containing nulls are counted, nulls are not included in the results. For example, there are 200 customers in the Customer_ Addresses table, and 30 have nulls in the Email_Address column. Doing a count using the Email_Address column will return a result of 170.