9.2. Comparison Functions and Operators
The usual comparison operators are available, as shown in Table 9.1 .
Table 9.1. Comparison Operators
Operator  Description 

datatype
<
datatype
→
boolean

Less than 
datatype
>
datatype
→
boolean

Greater than 
datatype
<=
datatype
→
boolean

Less than or equal to 
datatype
>=
datatype
→
boolean

Greater than or equal to 
datatype
=
datatype
→
boolean

Equal 
datatype
<>
datatype
→
boolean

Not equal 
datatype
!=
datatype
→
boolean

Not equal 
Note
<>
is the standard SQL notation for
"
not
equal
"
.
!=
is an alias, which is converted
to
<>
at a very early stage of parsing.
Hence, it is not possible to implement
!=
and
<>
operators that do different things.
These comparison operators are available for all builtin data types that have a natural ordering, including numeric, string, and date/time types. In addition, arrays, composite types, and ranges can be compared if their component data types are comparable.
It is usually possible to compare values of related data
types as well; for example
integer
>
bigint
will work. Some cases of this sort are implemented
directly by
"
crosstype
"
comparison operators, but if no
such operator is available, the parser will coerce the lessgeneral type
to the moregeneral type and apply the latter's comparison operator.
As shown above, all comparison operators are binary operators that
return values of type
boolean
. Thus, expressions like
1 < 2 < 3
are not valid (because there is
no
<
operator to compare a Boolean value with
3
). Use the
BETWEEN
predicates
shown below to perform range tests.
There are also some comparison predicates, as shown in Table 9.2 . These behave much like operators, but have special syntax mandated by the SQL standard.
Table 9.2. Comparison Predicates
Predicate Description Example(s) 

Between (inclusive of the range endpoints).

Not between (the negation of

Between, after sorting the two endpoint values.

Not between, after sorting the two endpoint values.

Not equal, treating null as a comparable value.

Equal, treating null as a comparable value.

Test whether value is null.

Test whether value is not null.

Test whether value is null (nonstandard syntax). 
Test whether value is not null (nonstandard syntax). 
Test whether boolean expression yields true.

Test whether boolean expression yields false or unknown.

Test whether boolean expression yields false.

Test whether boolean expression yields true or unknown.

Test whether boolean expression yields unknown.

Test whether boolean expression yields true or false.

The
BETWEEN
predicate simplifies range tests:
a
BETWEENx
ANDy
is equivalent to
a
>=x
ANDa
<=y
Notice that
BETWEEN
treats the endpoint values as included
in the range.
BETWEEN SYMMETRIC
is like
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.
If it is not, those two arguments are automatically swapped, so that
a nonempty range is always implied.
The various variants of
BETWEEN
are implemented in
terms of the ordinary comparison operators, and therefore will work for
any data type(s) that can be compared.
Note
The use of
AND
in the
BETWEEN
syntax creates an ambiguity with the use of
AND
as a
logical operator. To resolve this, only a limited set of expression
types are allowed as the second argument of a
BETWEEN
clause. If you need to write a more complex subexpression
in
BETWEEN
, write parentheses around the
subexpression.
Ordinary comparison operators yield null (signifying
"
unknown
"
),
not true or false, when either input is null. For example,
7 = NULL
yields null, as does
7 <> NULL
. When
this behavior is not suitable, use the
IS [
NOT
] DISTINCT FROM
predicates:
a
IS DISTINCT FROMb
a
IS NOT DISTINCT FROMb
For nonnull inputs,
IS DISTINCT FROM
is
the same as the
<>
operator. However, if both
inputs are null it returns false, and if only one input is
null it returns true. Similarly,
IS NOT DISTINCT
FROM
is identical to
=
for nonnull
inputs, but it returns true when both inputs are null, and false when only
one input is null. Thus, these predicates effectively act as though null
were a normal data value, rather than
"
unknown
"
.
To check whether a value is or is not null, use the predicates:
expression
IS NULLexpression
IS NOT NULL
or the equivalent, but nonstandard, predicates:
expression
ISNULLexpression
NOTNULL
Do
not
write
because
expression
= NULL
NULL
is not
"
equal to
"
NULL
. (The null value represents an unknown value,
and it is not known whether two unknown values are equal.)
Tip
Some applications might expect that
returns true if
expression
= NULL
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
.
If the
expression
is rowvalued, 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 nonnull
and all the row's fields are nonnull. Because of this behavior,
IS NULL
and
IS NOT NULL
do not always return
inverse results for rowvalued expressions; in particular, a rowvalued
expression that contains both null and nonnull fields will return false
for both tests. In some cases, it may be preferable to
write
row
IS DISTINCT FROM NULL
or
row
IS NOT DISTINCT FROM NULL
,
which will simply check whether the overall row value is null without any
additional tests on the row fields.
Boolean values can also be tested using the predicates
boolean_expression
IS TRUEboolean_expression
IS NOT TRUEboolean_expression
IS FALSEboolean_expression
IS NOT FALSEboolean_expression
IS UNKNOWNboolean_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.
Some comparisonrelated functions are also available, as shown in Table 9.3 .
Table 9.3. Comparison Functions