Logical Operators

Logical operators let you establish conditions and test their results.

Operator Unary/Binary Description Operands
NOT U Logical negation Boolean
AND B Conjunction Boolean
OR B Disjunction Boolean
IS B Logical assertion Boolean
IS NOT UNKNOWN U Negated unknown comparison: IS NOT UNKNOWN Boolean
IS NULL U Null comparison: IS NULL Any
IS NOT NULL U Negated null comparison: IS NOT NULL Any
= B Equality Any
!= B Inequality Any
<> B Inequality Any
> B Greater than Ordered types (Numeric, String, Date, Time)
>= B Greater than or equal to (not less than) Ordered types
< B Less than Ordered types
<= B Less than or equal to (not more than) Ordered types
BETWEEN Ternary Range comparison:col1 BETWEEN expr1 AND expr2 Ordered types
IS DISTINCT FROM B Distinction Any
IS NOT DISTINCT FROM B Negated distinction Any

Three State Boolean Logic

SQL boolean values have three possible states rather than the usual two: TRUE, FALSE, and UNKNOWN, the last of which is equivalent to a boolean NULL. TRUE and FALSE operands generally function according to normal two-state boolean logic, but additional rules apply when pairing them with UNKNOWN operands, as the tables that follow will show. Note: UNKOWN represents “maybe TRUE, maybe FALSE” or, to put it another way, “not definitely TRUE and not definitely FALSE.” This understanding may help you clarify why some of the expressions in the tables evaluate as they do.

Negation (NOT):

Operation Result
NOT TRUE FALSE
NOT FALSE TRUE
NOT UNKNOWN UNKNOWN

Conjunction (AND):

Operation Result
TRUE AND TRUE TRUE
TRUE AND FALSE FALSE
TRUE AND UNKNOWN UNKNOWN
FALSE AND TRUE FALSE
FALSE AND FALSE FALSE
FALSE AND UNKNOWN FALSE
UNKNOWN AND TRUE UNKNOWN
UNKNOWN AND FALSE FALSE
UNKNOWN AND UNKNOWN UNKNOWN

Disjunction (OR):

Operation Result
TRUE OR TRUE TRUE
TRUE OR FALSE TRUE
TRUE OR UNKNOWN TRUE
FALSE OR TRUE TRUE
FALSE OR FALSE FALSE
FALSE OR UNKNOWN UNKNOWN
UNKNOWN OR TRUE TRUE
UNKNOWN OR FALSE UNKNOWN
UNKNOWN OR UNKNOWN UNKNOWN

Assertion (IS):

Operation Result
TRUE IS TRUE TRUE
TRUE IS FALSE FALSE
TRUE IS UNKNOWN FALSE
FALSE IS TRUE FALSE
FALSE IS FALSE TRUE
FALSE IS UNKNOWN FALSE
UNKNOWN IS TRUE FALSE
UNKNOWN IS FALSE FALSE
UNKNOWN IS UNKNOWN TRUE

IS NOT UNKNOWN:

Operation Result
TRUE IS NOT UNKNOWN TRUE
FALSE IS NOT UNKNOWN TRUE
UNKNOWN IS NOT UNKNOWN FALSE

IS NOT UNKNOWN is a special operator in and of itself. The expression “x IS NOT UNKNOWN” is equivalent to “(x IS TRUE) OR (x IS FALSE)”, not “x IS (NOT UNKNOWN)”. Thus, substituting in the table above:

x Operation Result Result of substituting for x in “(x IS TRUE) OR (x IS FALSE)”
TRUE TRUE IS NOT UNKNOWN TRUE becomes ”(TRUE IS TRUE) OR (TRUE IS FALSE)” – hence TRUE
FALSE FALSE IS NOT UNKNOWN TRUE becomes ”(FALSE IS TRUE) OR (FALSE IS FALSE)” – hence TRUE
UNKNOWN UNKNOWN IS NOT UNKNOWN FALSE becomes ”(UNKNOWN IS TRUE) OR (UNKNOWN IS FALSE)” – hence FALSE,since UNKNOWN is neither TRUE not FALSE

Since IS NOT UNKNOWN is a special operator, the operations above are not transitive around the word IS:

Operation Result
NOT UNKNOWN IS TRUE FALSE
NOT UNKNOWN IS FALSE FALSE
NOT UNKNOWN IS UNKNOWN TRUE

IS NULL and IS NOT NULL

Operation Result
UNKNOWN IS NULL TRUE
UNKNOWN IS NOT NULL FALSE
NULL IS NULL TRUE
NULL IS NOT NULL FALSE

IS DISTINCT FROM and IS NOT DISTINCT FROM

Operation Result
UNKNOWN IS DISTINCT FROM TRUE TRUE
UNKNOWN IS DISTINCT FROM FALSE TRUE
UNKNOWN IS DISTINCT FROM UNKNOWN FALSE
UNKNOWN IS NOT DISTINCT FROM TRUE FALSE
UNKNOWN IS NOT DISTINCT FROM FALSE FALSE
UNKNOWN IS NOT DISTINCT FROM UNKNOWN TRUE

Informally, “x IS DISTINCT FROM y” is similar to “x <> y”, except that it is true even when either x or y (but not both) is NULL. “DISTINCT FROM” is the opposite of identical, whose usual meaning is that a value (true, false, or unknown) is identical to itself, and distinct from every other value. The IS and IS NOT operators treat UNKOWN in a special way, because it represents “maybe TRUE, maybe FALSE”.

Other logical operators:

For all other operators, passing a NULL or UNKNOWN operand will cause the result to be UNKNOWN (which is the same as NULL).

Examples:

Operation Result
TRUE AND CAST( NULL AS BOOLEAN) UNKNOWN
FALSE AND CAST( NULL AS BOOLEAN) FALSE
1 > 2 FALSE
1 < 2 TRUE
‘foo’ = ‘bar’ FALSE
‘foo’ <> ‘bar’ TRUE
‘foo’ <= ‘bar’ FALSE
‘foo’ <= ‘bar’ TRUE
3 BETWEEN 1 AND 5 TRUE
1 BETWEEN 3 AND 5 FALSE
3 BETWEEN 3 AND 5 TRUE
5 BETWEEN 3 AND 5 TRUE
1 IS DISTINCT FROM 1.0 FALSE
CAST( NULL AS INTEGER ) IS NOT DISTINCT FROM CAST (NULL AS INTEGER) TRUE