Table 3-1. Comparison Operators
Operator | Description |
---|
< | less than |
> | greater than |
<= | less than or equal to |
>= | greater than or equal to |
= | equal |
<> or != | not equal |
| 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.
is equivalent to
Similarly,
is equivalent to
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 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.)
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 (for example,
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.