The usual comparison operators are available, shown in Table 6-1.
Table 6-1. 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".