The usual comparison operators are available, shown in Table 9.1, “Comparison Operators”.
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 |
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
BETWEENx
ANDy
is equivalent to
a
>=x
ANDa
<=y
Similarly,
a
NOT BETWEENx
ANDy
is equivalent to
a
<x
ORa
>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 NULLexpression
IS NOT NULL
or the equivalent, but nonstandard, constructs
expression
ISNULLexpression
NOTNULL
Do not write
because expression
= NULLNULL
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.
Some applications may expect that
returns true if expression
= NULLexpression
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.
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 FROMexpression
expression
IS NOT DISTINCT FROMexpression
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 TRUEexpression
IS NOT TRUEexpression
IS FALSEexpression
IS NOT FALSEexpression
IS UNKNOWNexpression
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.