Identify NULL values

Use the IS NULL or IS NOT NULL option to check for NULL values. A NULL value represents either the absence of data or an unknown value. A NULL value is not the same as a zero or a blank.

The following query returns all rows that have a null paid_date, as the result shows.
Figure 1: Query
SELECT order_num, customer_num, po_num, ship_date
   FROM orders
   WHERE paid_date IS NULL
   ORDER BY customer_num;
Figure 2: Query result
order_num  customer_num  po_num   ship_date  

     1004           106  8006     05/30/1998
     1006           112  Q13557           
     1007           117  278693   06/05/1998
     1012           117  278701   06/29/1998
     1016           119  PC6782   07/12/1998
     1017           120  DM354331 07/13/1998