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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
Do not use expression = NULL because NULL is not "equal to" NULL. (NULL represents an unknown value, so it is not known whether two unknown values are equal.) Postgres presently converts x = NULL clauses to x IS NULL to allow some broken client applications (such as Microsoft Access) to work, but this may be discontinued in a future release.

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