PostgreSQL 8.3.1 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. `BETWEEN SYMMETRIC` is the same as `BETWEEN` except there is no requirement that the argument to the left of `AND` be less than or equal to the argument on the right; the proper range is automatically determined.

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.) This behavior conforms to the SQL standard.

> Tip: Some applications might expect that ``expression` = NULL` returns true if `expression` evaluates to the null value. It is highly recommended that these applications be modified to comply with the SQL standard. However, if that cannot be done the transform_null_equals configuration variable is available. If it is enabled, PostgreSQL will convert `x = NULL` clauses to `x IS NULL`. This was the default behavior in PostgreSQL releases 6.5 through 7.1.

> Note: If the `expression` is row-valued, then `IS NULL` is true when the row expression itself is null or when all the row's fields are null, while `IS NOT NULL` is true when the row expression itself is non-null and all the row's fields are non-null. This definition conforms to the SQL standard, and is a change from the inconsistent behavior exhibited by PostgreSQL versions prior to 8.2.

The ordinary comparison operators yield null (signifying "unknown") when either input is null. Another way to do comparisons is with the `IS [ NOT ] DISTINCT FROM` construct:

    expression IS DISTINCT FROM expression
    expression IS NOT DISTINCT FROM expression

For non-null inputs, `IS DISTINCT FROM` is the same as the `<>` operator. However, when both inputs are null it will return false, and when just one input is null it will return 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 constructs effectively act as though null were a normal data value, rather than "unknown".

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 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.

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