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 Data Types and SubtypesData Definition (DDL) Statements 
Boolean IS [NOT]
Syntax
<value> IS [NOT] { TRUE | FALSE | UNKNOWN }

The IS predicate with Boolean literal values checks if the expression on the left side matches the Boolean value on the right side.The expression on the left side must be of type BOOLEAN, otherwise an exception is raised.

The IS [NOT] UNKNOWN is equivalent to IS [NOT] NULL.

Note

The right side of the predicate only accepts the literals TRUE, FALSE, UNKNOWN, and NULL.It does not accept expressions.
Using the IS predicate with a Boolean data type
-- Checking FALSE value
SELECT * FROM TBOOL WHERE BVAL IS FALSE;

ID            BVAL
============= =======
2             <false>

-- Checking UNKNOWN value
SELECT * FROM TBOOL WHERE BVAL IS UNKNOWN;

ID            BVAL
============= =======
3             <null>
See also

[fblangref50-commons-isnotnull]

IS [NOT] NULL
Syntax
<value> IS [NOT] NULL

Since NULL is not a value, these operators are not comparison operators.The IS [NOT] NULL predicate tests that the expression on the left side has a value (IS NOT NULL) or has no value (IS NULL).

Example

Search for sales entries that have no shipment date set for them:

SELECT * FROM SALES
WHERE SHIP_DATE IS NULL;
Note
Note regarding the IS predicates

The IS predicates take precedence above the others.