PostgreSQL 7.4beta4 Documentation
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# 9.2. Comparison Operators

The usual comparison operators are available, shown in Table 9-1.

Table 9-1. Comparison Operators

Operator | Description
--- | ---
< | less than
> | greater than
<= | less than or equal to
>= | greater than or equal to
= | equal
<> or != | not equal

> Note: The != operator is converted to <> in the parser stage. It is not possible to implement != and <> operators that do different things.

Comparison operators are available for all data types where this makes sense. 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).

In addition to the comparison operators, the special `BETWEEN` construct is available.

    a BETWEEN x AND y

is equivalent to

    a >= x AND a <= y

Similarly,

    a NOT BETWEEN x AND y

is equivalent to

    a < x OR a > y

There is no difference between the two respective forms apart from the CPU cycles required to rewrite the first one into the second one internally.

To check whether a value is or is not null, use the constructs

    expression IS NULL
    expression IS NOT NULL

or the equivalent, but nonstandard, constructs

    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.)

Some applications may (incorrectly) require that expression = NULL returns true if expression evaluates to the null value. To support these applications, the run-time option transform_null_equals can be turned on (e.g., SET transform_null_equals TO ON;). PostgreSQL will then convert x = NULL clauses to x IS NULL. This was the default behavior in releases 6.5 through 7.1.

Boolean values can also be tested using the constructs

    expression IS TRUE
    expression IS NOT TRUE
    expression IS FALSE
    expression IS NOT FALSE
    expression IS UNKNOWN
    expression IS NOT UNKNOWN

These are similar to IS NULL in that they 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".

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