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Comparison Operators

Comparison operators compare a value against another value.

Usual comparison operators

The usual comparison operators are available:

<less than
>greater than
<=less than or equal to
>=greater than or equal to
<> or !=not equal

Comparison operators are available for all relevant data types. All comparison operators are binary operators that return values of type boolean. Expressions like 1 < 2 < 3 are not valid (because there is no < operator to compare a Boolean value with 3).


The BETWEEN predicate simplifies range tests

<a> BETWEEN <x> AND <y>
<a> NOT BETWEEN <x> AND <y>

a BETWEEN x AND y is equivalent to a >= x AND a <= y. Notice that BETWEEN treats the endpoint values as included in the range. NOT BETWEEN does the opposite comparison: a NOT BETWEEN x AND y is equivalent to a < x OR a > y.

NULL-sensitive comparison operators

While the normal comparison operators and BETWEEN ignore NULLs, we sometimes want different NULL behavior. For this purpose, the SQL standard provides the following comparison functions. These behave much like operators but have special syntax mandated by the SQL standard.

<expression> IS NULL
<expression> IS NOT NULL
<expression> ISNULL -- nonstandard syntax
<expression> NOTNULL -- nonstandard syntax
<boolean_expression> IS TRUE
<boolean_expression> IS NOT TRUE -- is false or unknown
<boolean_expression> IS FALSE
<boolean_expression> IS NOT FALSE -- is true or unknown
<boolean_expression> IS UNKNOWN
<boolean_expression> IS NOT UNKNOWN -- is true or false

Ordinary comparison operators yield null (signifying "unknown"), not true or false, when either input is null. For example, 7 = NULL yields null, as does 7 <> NULL. When this behavior is not suitable, use the IS NOT DISTINCT FROM predicates: a IS DISTINCT FROM b, a IS NOT DISTINCT FROM b. For non-null inputs, IS DISTINCT FROM is the same as the <> operator. However, if both inputs are null it returns false, and if only one input is null it returns true. Similarly, IS NOT DISTINCT FROM is identical to = for non-null inputs, but it returns true when both inputs are null, and false when only one input is null. Thus, these predicates effectively act as though null were a normal data value, rather than "unknown".

To check whether a value is or is not null, use the predicates expression IS NULL, expression IS NOT NULL or the equivalent, but nonstandard, predicates: expression ISNULL, expression NOTNULL.

Do not write expression = NULL because NULL is not "equal to" NULL. (The null value represents an unknown value, and it is not known whether two unknown values are equal.)

Boolean values can also be tested using the predicates IS TRUE, IS NOT TRUE, IS FALSE, IS NOT FALSE, IS UNKNOWN, and IS NOT UNKNOWN. These will always return true or false, never a null value, even when the operand is null. A null input is treated as the logical value "unknown". Notice that IS UNKNOWN and IS NOT UNKNOWN are effectively the same as IS NULL and IS NOT NULL, respectively, except that the input expression must be of Boolean type.


IN and NOT IN compare an expression against multiple values. They use the syntax

<expression> IN ( <value>, ... )
<expression> NOT IN ( <value>, ... )

The right-hand side is a parenthesized list of scalar expressions. The value from the left-hand side is compared against each of the expressions from the right-hand side.

For IN, the result is "true" if the left-hand expression's result is equal to any of the right-hand expressions. This is a shorthand notation for expression = value1 OR expression = value2 OR ...

For NOT IN the result is "true" if the left-hand expression's result is unequal to all of the right-hand expressions. This is a shorthand notation for expression <> value1 AND expression <> value2 AND ...

Note that if the left-hand expression yields null, or if there are no equal right-hand values and at least one right-hand expression yields null, the result of the IN/NOT IN construct will be null, not false. This is in accordance with SQL's normal rules for Boolean combinations of null values.

IN and NOT IN are syntactically related to the subquery forms but do not involve subqueries.


x NOT IN y is equivalent to NOT (x IN y) in all cases. However, null values are much more likely to trip up the novice when working with NOT IN than when working with IN. It is best to express your condition positively if possible.