Relational-Operator Condition

A relational operator compares two expressions quantitatively.

For a list of the supported relational operators and their descriptions, see Relational Operator.

The following examples show some relational-operator conditions: 
city[1,3] = 'San'

o.order_date > '6/12/98'

WEEKDAY(paid_date) = WEEKDAY(CURRENT- (31 UNITS DAY))

YEAR(ship_date) < YEAR (TODAY)

quantity <= 3

customer_num <> 105

customer_num != 105

Operands in relational operator conditions cannot have UNKNOWN or NULL values. If an expression within the condition has an UNKNOWN value because it references an uninitialized variable, the database server raises an exception.

Conditions testing for NULL values

If any expression within the condition evaluates to NULL, the condition cannot be true, unless you are explicitly testing for NULL by using the IS NULL operator. For example, if the paid_date column has a NULL value, then neither of the following queries can retrieve that row: 
SELECT customer_num, order_date FROM orders
   WHERE paid_date = '';

SELECT customer_num, order_date FROM orders
   WHERE NOT (paid_date !='');
You must use the IS NULL operator to test for a NULL value, as the next example shows. 
SELECT customer_num, order_date FROM orders
   WHERE paid_date IS NULL;
The IS NULL operator and its logical inverse, the IS NOT NULL operator, are described in IS NULL and IS NOT NULL Conditions.