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 
<>
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.
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
= NULLNULL
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 might 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
.
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