PostgreSQL 7.2.1 Documentation
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# 4.2. Comparison Operators

Table 4-1. 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 less standard, constructs

    expression ISNULL
    expression NOTNULL

Do not write ``expression` = NULL` because NULL is not "equal to" NULL. (NULL 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 NULL, even when the operand is NULL. A NULL input is treated as the logical value UNKNOWN.

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